Johnson, 28, has seen his season blighted by injury. A dislocated collarbone and a groin strain have limited him to just 13 appearances for the club this term. The news is a further blow for Fulham manager Roy Hodgson who already has forwards Clint Dempsey and Zoltan Gera suffering with knee complaints. USA international Dempsey has knee ligament damage while Hungarian Gera took a blow to his knee in training on Tuesday. Johnson's injury comes at a bad time for Fulham, as Hodgson allowed Diomansy Kamara to join Celtic on loan believing Johnson was progressing well from his injury. At the club's press conference at Motspur Park, Hodgson said: "AJ's going to have an exploratory operation on his knee. "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. "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." Hodgson's team play Portsmouth in the Premier League on Wednesday night hoping to end an unwelcome record of five straight defeats. Hodgson added: "Relegation has to be an issue for everybody who isn't totally free of it. "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."
