Manchester United and England striker Wayne Rooney has had a scan on the ankle injury he suffered in Tuesday's 2-1 loss to Bayern Munich.
He turned his right ankle in the build-up to Bayern's late winner in the Champions League, limping off the pitch and leaving the stadium on crutches.
Rooney, 24, is a huge doubt for Saturday's game with rivals Chelsea but the extent of the injury is unclear.
United boss Sir Alex Ferguson has said the injury was "not terribly serious".
However, Rooney's situation will be a concern for Ferguson, whose side lead Chelsea by a point in the battle for the Premier League title, and England coach Fabio Capello, coming 10 weeks before the start of the World Cup finals in South Africa.
Rooney leaves Munich on crutches
"He has obviously hurt his ankle," added Ferguson after the game on Tuesday. "It is too early to say. I don't think it's terribly serious but we'll wait and see."
Rooney has been in scintillating form this season, scoring 34 goals in all competitions including 26 in 29 league matches, and is regarded as being key to England's hopes of World Cup glory.
It was an eventful night for the striker, who opened the scoring for United with a close-range volley after only 66 seconds before deflecting in Franck Ribery's free-kick for Bayern's equaliser.
And in the dying moments of the game, Rooney then appeared to be in severe pain after landing awkwardly on his right ankle. He left the ground on crutches and with his foot in a protective cast.
The United website said that he "did not travel back on the team bus from the Allianz Arena, instead departing separately with security staff".
Rooney leaves the Allianz Arena with a cast on his foot
When asked if Rooney would feature against Chelsea in Saturday's 1245 BST kick-off at Old Trafford, Ferguson added: "I can't answer that, we will know better on Wednesday."
Manchester United take on Bayern in the Champions League quarter-final second leg on Wednesday 7 April.
Any extended spell on the sidelines would be a major blow for United as the season reaches its climax.
But if Rooney is ruled out, Ferguson says he has full confidence that 18-year-old striker Federico Macheda will be able to step up.
Macheda, who scored a crucial winner on his United debut against Aston Villa last season, returned to Premier League action last weekend after recovering from injury.
"Macheda's had a terrible season with injuries," Ferguson told the club's official website before the Bayern match. "But he made a return against Bolton last Saturday and we were absolutely delighted.
"He's one of the most exceptional young strikers I've seen in my time at United. He has every chance of taking part in big matches. His ability around the box for a young player is outstanding."
Capello will also be hoping that Rooney's injury is not too serious, with England's next international against Mexico on 24 May and the opening game of the World Cup taking place on 12 June against the United States.
The injury scare was the lead in Wednesday's newspapers, with headlines such as "England's Worst Nightmare", "Get Well Soon" and simply "Pray" splashed over the back pages.
Rooney also had an injury scare ahead of the 2006 World Cup after fracturing a metatarsal bone in his right foot during a 3-0 defeat at Chelsea.
He did not play in the opening game of that tournament, despite being named on the bench, but featured in all four of England's subsequent games in Germany.
In Euro 2004, Rooney limped out of the quarter-final defeat by Portugal after breaking a different metatarsal in his right foot.
And Bayern coach Louis van Gaal said he hoped that Rooney's injury was not too serious.
"Rooney is very important for Manchester United," he said. "He is their most important player.
MAN UTD'S APRIL FIXTURES
3rd: Chelsea (H)
7th: Bayern Munich (H)
11th: Blackburn (A)
17th: Man City (A)
24th: Tottenham (H)
"Rooney is so important for Man Utd and I hope for them that he is not seriously injured."
Capello's year has started with several off-the-field setbacks, with injuries to a host of key players and the row between former captain John Terry and defender Wayne Bridge dominating the headlines.
Terry was stripped of the captaincy after allegations of an affair with Bridge's former girlfriend Vanessa Perroncel. Bridge has subsequently opted not to be chosen for England duty.
Meanwhile, another former captain David Beckham has been ruled out of the World Cup after rupturing an Achilles tendon playing for AC Milan.
Chelsea left-back Ashley Cole is battling to be fit before the end of the season after suffering a broken ankle while winger Aaron Lennon has been struggling with groin muscle tears.
England need Wayne Rooney - Bruce
Striker Jermain Defoe is back in training after suffering a tear to his hamstring while defenders Joleon Lescott and Wes Brown are both out injured.
Rooney's injury compounded a bad night for United and Ferguson conceded his side had not produced their best against Bayern.
"We gave the ball away too easily from minute one," he said. "Our possession wasn't our standard and that's something we pride ourselves on.
"It's an advantage to score away from home and we had outstanding chances to kill the tie, twice with Rooney, once in the first half and once in the second half.
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