The England international picked up the problem in this month's 2-1 defeat by Manchester City and attended Wednesday's 2-2 draw at West Ham United on crutches.

"Ashley is out for the season," Ferguson, whose side can secure the Premier League title on Monday if they beat Aston Villa and second-placed Manchester City lose at Tottenham Hotspur on Sunday, told MUTV.

"It's unfortunate and it's a bad blow. We thought it'd be maybe two or three weeks but now he's been down to the specialist and that's why you saw him at the game on Wednesday.

"We took him down to a specialist in London and the prognosis was not very good at all. We're going to lose him for the rest of the season."

United have 81 points from 33 games with City on 68 from 32.

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