Nani, before his departure, helped set up United's swift response when Ryan Giggs returned his cross into the penalty area and James Collins deflected the ball into his own net. It was the 10th own goal in United's favour this season.
Despite being reduced to 10 men, United were the better side and were inspired by another virtuoso display from Wayne Rooney, who covered every blade of grass at Villa Park to compensate for Nani's loss.
Villa faded after an excellent opening and United controlled the remainder of proceedings, with Michael Carrick and Rooney bringing the best out of Villa keeper Brad Friedel.
United boss Sir Alex Ferguson may be frustrated by his side's failure to get the maximum return from their performance, but there was much to admire in their resilience and the quality of Rooney and they will have been lifted by news of Chelsea's defeat at Everton.
O'Neill 'couldn't be more pleased' with Villa
As well as Nani's dismissal, Ferguson had another concern with Giggs going off near the end clutching his arm.
Villa gave a rare start to gifted youngster Fabian Delph, the summer signing from Leeds and he excelled as Martin O'Neill's side took charge in the opening exchanges.
United struggled to cope with Villa's quality of delivery from the flanks, so it was no surprise that their failure to deal with Downing's cross led to the opener after 19 minutes.
Rafael could only partially clear, but Cuellar still did well to get enough power behind a lofted, looping header to beat Edwin van der Sar.
Villa's lead lasted only four minutes, and the champions were indebted to a large slice of good fortune to restore equality.
Nani found Giggs at the far post, and his volley was flying across the face of goal until it struck Collins and flew past Friedel.
It was Nani's last positive contribution was he was shown the red card after 28 minutes.
He attempted to win possession in a tussle with Petrov by launching a senseless, two-footed lunge and left referee Peter Walton with the easiest of decisions to send him off.
As so often, the side with a numerical disadvantage seemed to adapt quicker to the change in circumstances and United suddenly looked the more controlled side, with Friedel turning Carrick's shot over the top.
Phelan backs Nani red card
Ferguson made a change at the interval, withdrawing Paul Scholes and introducing Antonio Valencia, who almost made an instant impact with a rising effort that flew just off target.
James Milner tested Van der Sar before O'Neill made a change of his own, somewhat surprisingly replacing Delph with the giant figure of John Carew.
United were forced into another change with 17 minutes left when Giggs hurt his arm in an innocuous incident, allowing Dimitar Berbatov to enter the action.
Rooney was starting to exert his quality on proceedings and he brought another fine save from Friedel with 15 minutes left as he drove in a powerful shot from the edge of the area.
It was the last serious threat from either side and they were both forced to settle for a point ahead of their big Wembley date.
Aston Villa manager Martin O'Neill: "I thought we could maybe take advantage but we were a bit tired in the second half and we didn't create enough to cause them undue worry.
"But they haven't been gallivanting around Europe winning Champions Leagues and titles without being a top side.
"I thought it was a red card, when you go in two-footed nowadays there is a decent chance the referee will show a red card."
Manchester United manager Sir Alex Ferguson: "It was an important night and, with Everton winning against Chelsea, we have gained one point and one goal.
"Nani is not a malicious player but he has gone in with his foot raised off the ground and we cannot complain about it.
"We should have had a foul seconds before when Agbonlahor came in with his foot up on Wes Brown's ribs.
"That was disappointing but we have no argument at all with the red card, it was a naive tackle."
90:00+0:42 The ball is delivered by Darren Fletcher, Luke Young manages to make a clearance.
89:37 Foul by Wayne Rooney on Gabriel Agbonlahor, free kick awarded. Free kick taken by Stewart Downing.
88:18 Antonio Valencia fouled by Carlos Cuellar, the ref awards a free kick. Free kick taken by Edwin Van der Sar.
87:07 Darren Fletcher fouled by James Milner, the ref awards a free kick. Darren Fletcher takes the free kick.
85:48 The ball is swung over by Carlos Cuellar, Headed effort on goal by Steve Sidwell from just inside the box goes harmlessly
over the target.
83:50 Carlos Cuellar produces a cross.
77:57 Rafael Da Silva crosses the ball, James Collins manages to make a clearance.
77:15 Centre by Antonio Valencia, clearance by Carlos Cuellar.
74:27 Effort on goal by Wayne Rooney from 30 yards. Save made by Brad Friedel. Wayne Rooney takes a corner. Wayne Rooney takes a
Correction - 71:37 Free kick crossed by Darren Fletcher, John Carew manages to make a clearance.
Correction - 71:37 Substitution Ryan Giggs leaves the field to be replaced by Dimitar Berbatov.
Correction - 71:37 Free kick awarded for an unfair challenge on Rafael Da Silva by Steve Sidwell.
71:37 Darren Fletcher takes the direct free kick.
71:37 Substitution Dimitar Berbatov replaces Ryan Giggs.
71:37 Steve Sidwell gives away a free kick for an unfair challenge on Rafael Da Silva.
69:55 The official flags Antonio Valencia offside. Brad Friedel restarts play with the free kick.
68:33 Antonio Valencia gives away a free kick for an unfair challenge on James Collins. Brad Friedel takes the direct free kick.
66:08 James Milner concedes a free kick for a foul on Michael Carrick. Edwin Van der Sar restarts play with the free kick.
65:12 Shot by Darren Fletcher. Carlos Cuellar gets a block in.
64:38 Unfair challenge on Ryan Giggs by James Milner results in a free kick. Jonathan Evans takes the free kick.
63:05 Free kick taken by Richard Dunne.
63:05 Substitution Steve Sidwell comes on in place of Stiliyan Petrov.
63:05 Foul by Antonio Valencia on Luke Young, free kick awarded.
62:09 Effort on goal by Wayne Rooney from just inside the penalty area goes harmlessly over the target.
Correction - 61:19 The ball is delivered by Antonio Valencia, Brad Friedel makes a save. The assistant referee flags for offside against Gabriel
Agbonlahor. Jonathan Evans takes the free kick.
61:19 The ball is swung over by Antonio Valencia, Brad Friedel makes a save. The assistant referee flags for offside against Gabriel
Agbonlahor. Jonathan Evans takes the indirect free kick.
60:44 Stewart Downing delivers the ball, Antonio Valencia manages to make a clearance.
60:01 Free kick awarded for a foul by Rafael Da Silva on Ashley Young. Direct free kick taken by James Milner.
59:28 Effort from the edge of the area by Wayne Rooney goes wide of the right-hand upright.
58:41 The ball is delivered by Ashley Young, save by Edwin Van der Sar.
57:51 Substitution Fabian Delph leaves the field to be replaced by John Carew.
57:02 Effort on goal by James Milner from 18 yards. Edwin Van der Sar makes a save.
53:57 Outswinging corner taken from the left by-line by Ryan Giggs, clearance made by Gabriel Agbonlahor.
51:44 Ryan Giggs sends in a cross, Effort on goal by Antonio Valencia from just inside the area clears the crossbar.
48:49 Luke Young challenges Rafael Da Silva unfairly and gives away a free kick. Edwin Van der Sar takes the direct free kick.
46:12 The ball is sent over by Darren Fletcher.
45:05 Ryan Giggs takes the outswinging corner, Header on goal by Darren Fletcher from inside the box goes harmlessly over the target.
45:01 The referee gets the second half underway.
45:01 Substitution Antonio Valencia replaces Paul Scholes.
45:00+3:07 It is the end of the first-half.
45:00+0:29 Shot from just outside the area by Stewart Downing goes over the bar.
42:28 Wayne Rooney concedes a free kick for a foul on Ashley Young. Brad Friedel restarts play with the free kick.
42:01 Rafael Da Silva fouled by James Milner, the ref awards a free kick. Free kick taken by Ryan Giggs.
38:54 Free kick awarded for an unfair challenge on Gabriel Agbonlahor by Wes Brown. Ashley Young fires a strike on goal direct from
the free kick, blocked by Wayne Rooney.
37:08 The assistant referee flags for offside against Wayne Rooney. James Milner restarts play with the free kick.
35:53 The official flags Ashley Young offside. Free kick taken by Rafael Da Silva.
34:59 The ball is swung over by Ryan Giggs, Effort on goal by Darren Fletcher from inside the penalty area goes harmlessly over
33:49 Outswinging corner taken by Ryan Giggs, Shot by Michael Carrick. Save made by Brad Friedel. Ryan Giggs takes a outswinging
corner from the left by-line played to the near post, clearance by Stiliyan Petrov.
33:20 The ball is swung over by James Milner, Paul Scholes manages to make a clearance.
32:52 Effort from 20 yards by Stiliyan Petrov. Blocked by Paul Scholes.
32:43 Luke Young produces a cross, clearance by Wes Brown.
30:56 Gabriel Agbonlahor gives away a free kick for an unfair challenge on Wes Brown. Edwin Van der Sar takes the free kick.
30:39 Stewart Downing produces a cross, clearance by Rafael Da Silva.
Correction - 28:38 Sent off The referee shows Luis Nani a red card for serious foul play.
28:38 Sent off The referee shows Luis Nani a red card.
28:25 Foul by Luis Nani on Stiliyan Petrov, free kick awarded. Free kick taken by Luke Young.
26:39 Wes Brown is penalised for a handball. James Milner delivers the ball from the free kick right-footed from left channel, clearance
by Wes Brown.
25:50 Booking Jonathan Evans booked.
25:43 Unfair challenge on Stiliyan Petrov by Jonathan Evans results in a free kick. James Collins takes the direct free kick.
22:03 GOAL - James Collins:Aston Villa 1 - 1 Man Utd James Collins puts the ball into his own goal from 6 yards. Aston Villa 1-1 Man Utd.
Correction - 21:57 Luis Nani produces a cross, Ryan Giggs takes a shot. James Collins gets a block in.
21:57 The ball is sent over by Luis Nani, Ryan Giggs takes a shot. Blocked by James Collins.
20:47 Paul Scholes takes a shot. Blocked by James Collins.
18:23 GOAL - Carlos Cuellar:Aston Villa 1 - 0 Man Utd Carlos Cuellar grabs a headed goal from inside the area. Aston Villa 1-0 Man Utd.
18:19 Stewart Downing delivers the ball, clearance made by Rafael Da Silva.
17:41 Shot by Ashley Young from 25 yards. Jonathan Evans gets a block in. Inswinging corner taken left-footed by Stewart Downing
played to the near post, save by Edwin Van der Sar.
17:13 Gabriel Agbonlahor is caught offside. Free kick taken by Jonathan Evans.
15:26 Stiliyan Petrov produces a cross.
15:15 Ashley Young produces a cross, Jonathan Evans manages to make a clearance.
14:51 The ball is crossed by Luke Young, Wes Brown makes a clearance.
12:09 Patrice Evra concedes a free kick for a foul on Ashley Young. Stewart Downing crosses the ball, Header from deep inside the
area by Carlos Cuellar goes over the bar.
11:56 James Milner takes a shot from 25 yards. Blocked by Jonathan Evans.
8:20 Shot by Ryan Giggs. Richard Dunne gets a block in. Corner taken by Luis Nani from the left by-line played to the near post,
Carlos Cuellar manages to make a clearance.
7:16 Carlos Cuellar challenges Paul Scholes unfairly and gives away a free kick. Shot comes in from Luis Nani from the free kick,
save by Brad Friedel.
Correction - 6:33 Effort from the edge of the penalty box by Stewart Downing goes wide of the right-hand upright.
6:33 Effort from the edge of the area by Stewart Downing goes wide of the left-hand upright.
4:29 Free kick awarded for an unfair challenge on Richard Dunne by Luis Nani. Fabian Delph takes the direct free kick.
3:09 Centre by Darren Fletcher.
0:55 The ball is sent over by James Milner, save made by Edwin Van der Sar.
0:00 The match gets underway.
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