Ferguson's side came from behind to win for the second game in succession
Manchester United manager Sir Alex Ferguson admitted to tactical errors as his side had to come from behind to beat Southampton in the FA Cup.
League One Saints bossed the first half of the fourth-round tie and led 1-0 at the break but United rallied and two second-half goals put them through.
"In fairness, I maybe meddled with the system somewhat," Ferguson said. "It wasn't working at all.
"It was a nothing first half and the goal, if anything, made us play."
It was predatory finishes from strikers Michael Owen and Javier Hernandez that overturned Richard Chaplow's fiercely driven opener for the Saints, but United were as much indebted to Ryan Giggs and Nani, who stepped up from the bench 13 minutes into the second half and provided the impetus for the comeback.
Giggs and Nani - who replaced fellow midfielders Anderson and Darron Gibson - provided the width that had been lacking from a starting midfield Ferguson had initially set up in a diamond formation.
This is what we were after. The vision is there for the football club. We want the stadium full every week and what we have got to do is work very hard to get out the division we are in so we can have atmospheres like this every week.
Southampton boss Nigel Adkins
"We tried a diamond in the middle of the pitch and, of course, Manchester United are used to playing with width all the time and I think that was a problem for us," Ferguson explained.
"We had some possession of the ball but we never really threatened. We had two or three shots from the edge of the box."
Ferguson said that the timing of Owen's goal - the striker's first for the club since September - was important to ensure the game did not get away from United.
"He only had one chance today: one chance, one goal," Ferguson added.
Owen felt his side's superior quality showed in the final 30 minutes.
"They played well - they're flying high in their division," Owen told ITV.
"In the FA Cup, no matter what standard you're playing against it's always really tough for the first 45 minutes, an hour.
"But they've got to run more to keep up with us. They tire and it's often the way that the better teams come good in the last half-hour.
"We played in the Carling Cup earlier in the season against Scunthorpe away and it was similar. They went a goal ahead and it was only in the last half an hour that we ran away with it."
Southampton manager Nigel Adkins felt his side put up a strong showing, but ultimately United's extra class told.
"I thought it was a cracking game of football," Adkins reflected. "We tried to pass the ball about and we deservedly went in 1-0 up at half time.
"In the second half, for the first 20 minutes, we had opportunities and that pass just went astray from us.
"In the end, we were playing against Manchester United and that bit of class told with the finishing in the end.
"They are top of the league, they're undefeated in the league, and they have always got ways they can change the game."
Adkins believes the Saints' performance demonstrates the progress the side is making towards gaining promotion back to the Championship.
"This is what we were after," he explained. "The vision is there for the football club. We want the stadium full every week and what we have got to do is work very hard to get out the division we are in so we can have atmospheres like this every week.
"I expect us to win the league. That is what I have come here to do. It is going to be a tough ask because there are a lot of other teams who want to win the league and get promoted."
Adkins also said that he expects highly-rated 17-year-old winger Alex Oxlade-Chamberlain - who showed glimpses of his ability against United - to remain at St Mary's for the rest of the season despite speculation linking him with Arsenal and Liverpool.
"We are not envisaging selling him, we're not looking to sell him," the Saints boss stated. "We don't want to sell him.
"I think it is beneficial for him to be playing regularly for Southampton in the Football League, giving him a chance to develop.
"Yes, he is well courted and you can see today that he has ability about him. Southampton Football Club are not looking to sell Alex Chamberlain.
"There is always going to be mass speculation; there has been throughout the transfer window.
"It shuts at 11 o'clock on Monday evening and there will be mass speculation all the way up until then.
"All we can do is control what we are able to, which is getting Alex ready with the group in preparation for a very big [league] game for this football club on Tuesday night at Exeter."
Ferguson also turned his mind towards his own side's Premier League encounter with Aston Villa on Tuesday for which he expects to have defenders Rio Ferdinand, Nemanja Vidic, Patrice Evra and Rafael - all of whom sat out Saturday's match - available to play.
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