Manchester United manager David Moyes has confirmed Marouane Fellaini will be out for up to six weeks following wrist surgery.
Moyes revealed Fellaini had an operation at the weekend after a troublesome back complaint failed to clear up as hoped.
"He had his operation on Saturday morning," said Moyes.
"We just couldn't get him recovered from his back. We thought it would be okay after 10 days but it wasn't getting any better so we made a quick decision to get the wrist done.
"We think it will be six weeks but there is a chance it could be shorter than that."
If Fellaini's recovery goes as scheduled, he will miss seven Premier League games, including the trip to Chelsea on 19 January, plus both legs of the Capital One Cup semi-final with Sunderland and the FA Cup third and fourth rounds.
Crucially, though, the Belgium midfielder will be back well ahead of the trip to Olympiacos on 25 February in the first leg of United's Champions League knockout round tie.
Moyes confirmed the back injury should recover with rest and he anticipated Fellaini, who was back in Manchester on Monday, would go on to show the form which got him a summer move from Everton in the first place.
"His back problem is going to take time. That is a muscle injury which wasn't healing," said Moyes.
"It is unlucky. He has not felt 100 per cent because of his wrist. He has been finding it difficult.
"I asked him to see if he could get over the Christmas period. He got the back injury against Everton and since then he has never really been right.
"I have no doubt he will eventually be a good player for us."