Danny Welbeck sparked an England Euro 2012 worry when he left the Etihad Stadium wearing a protective boot on his ankle following Manchester United's 1-0 defeat by Manchester City.
Welbeck was caught by a nasty challenge from Nigel de Jong in the second half, which sparked a furious touchline row between Sir Alex Ferguson and Roberto Mancini.
It is not known whether he sustained the injury in that incident.
However, if the damage is bad, it could have serious consequences for Roy Hodgson, who seemed likely to select Welbeck in his European Championship squad if, as expected, he is confirmed as Fabio Capello's replacement.
Ferguson was certainly not happy with De Jong's challenge, nor the manner in which Mancini was talking to fourth official Mike Jones.
Jones stood between the pair at one point during the row to prevent it spilling over before they were eventually pulled apart by members of their respective backroom teams.
"He refereed the game," said Ferguson.
"He was out on that touchline the whole game haranguing the referee, the fourth official and the linesmen.
"The minute I come off the bench for bad tackle by De Jong on Welbeck, he was out again.
"He can't have it both ways. He's been complaining about referees this season but he won't be complaining tonight that's for sure."
Ferguson will have no doubts over where United lost their Premier League title if City win their next two games.
The Red Devils will relinquish their crown if City win their final two matches, at Newcastle next Sunday and then at home to QPR a week later.
Vincent Kompany's header in first-half stoppage time did the damage, with United finding nothing in response.
But Ferguson concedes it was the loss of those two goals in the final seven minutes at Everton that really saw his team stumble, allowing City back into a title race they seemed out of when they were eight points behind just four matches ago.
"You can look back at all sorts of games but you've got to look at the Everton one," he said.
"That was a ridiculous performance for the last seven minutes to lose two goals. That's maybe cost us the league.
"We know we need to win two games of football, that's another way of looking at it. Of course, we're certainly not in the driving seat any more."
Manchester City - Pld 36 Pts 83 GD +61
May 6 - Newcastle A (1600 BST)
May 13 - QPR H (1500)
Manchester United - Pld 36 Pts 83 GD +53
May 6 - Swansea H (1330)
May 13 - Sunderland A (1500)