Everton have confirmed Tim Cahill will be out of action for between six and eight weeks with the knee injury he sustained playing against Aston Villa.
Cahill has been hugely important for Everton this season
The 26-year-old midfielder was stretchered off early in Saturday's 1-0 home defeat after a freak collision with team-mate Lee Carsley.
He is now expected to be sidelined possibly until the end of January.
"I can't put into words how much he'll be missed for the next six to eight weeks," said Everton boss David Moyes.
Moyes added: "He will do everything that he can to get back from this injury as quickly as possible, he will want to make sure that he is not out for eight weeks.
"He is tough and he will be pushing himself all the time. That is an asset.
"I think we were all a bit concerned that it could have been worse than it actually is. The problem is with his medial ligament and there is also a little bit of damage to the cartilage.
"He might have a slight tear. But we are thankful that it is not his cruciate as that would have been it for the season."
Everton physio Mick Rathbone said: "We had the scans on Monday and they confirmed what we saw clinically, which is that he has significant damage to his medial ligament.
"The ACL (anterior cruciate ligament) seems OK which was our fear.
"We would estimate it would be an absolute minimum of six weeks but probably going on for eight weeks for the medial ligament.
"We think and hope the cartilage is OK. What we will do is let it settle down for a few days then reassess. The surgeon may decide to have a quick look inside the knee at the cartilage but we are not really sure about that.
"I suppose everybody knew it was a serious injury; it was clear having looked at the video tape."