Roy Hodgson is worried that Andy Johnson could be out for the rest of the season and has voiced his regret over Diomansy Kamara's loan move to Celtic.
The Fulham manager has had to cope with injuries to some of his key players this term but Johnson had looked to be progressing well after a recent spell on the sidelines.
However, the striker will now have to undergo an exploratory knee operation to discover whether he requires further surgery that could rule him out for several months.
Hodgson hopes that will not be the case as he would like to welcome Johnson back into the fold before the end of Febrruary.
"AJ's going to have an exploratory operation on his knee," said Hodgson.
"If they find that the problem is as major as it could possibly be, then he would need an operation that would keep him out for the rest of the season.
"If we're lucky and the major operation is not needed, and it's something wrong with the cartilage, then he might be back earlier.
"At the moment we're talking three to four weeks, and the worst-case scenario is the rest of the season and him being fit to join us in pre-season."
Having allowed fellow forward Kamara to link up with Celtic, Johnson's injury will leave Hodgson short of attacking options.
The manager added: "One of the reasons why we let Diomansy go was because we didn't think the problem with Andy was anywhere near as grave.
"I was concerned that I would not be able to give Diomansy regular games, so when a club of Celtic's stature comes in for him, it is very difficult for me to say 'you have to stay'.
"Of course now I have let him go and have further news about Andy, I might be beginning to regret it, but you can only make decisions at the time.
"If you said to me, would I like him back here, then I would, but it's too late.
"Andy is shattered. He's devastated. If it's as serious as it could turn out to be, it needs to be put right for the start of next season."
Fulham's injuries are threatening to derail a promising campaign and Hodgson admits the priority is to end a run of five straight defeats against Portsmouth on Wednesday and beat the drop.
"Relegation has to be an issue for everybody who isn't totally free of it," he stated.
"You'd have to be up around 30 points to say it's no interest, but it doesn't occupy my thoughts.
"At the moment I can divorce performances from results, but if you keep losing you will get dragged down there and get into a relegation battle.
"I'm hoping that won't happen and in the not-too-distant future we will have a squad of players together who are more competitive than we currently are."