Fulham's on-loan midfielder Lucas Piazon suffered a broken leg in Tuesday's 0-0 draw at Leeds.
Piazon, 23, was substituted in the 35th minute at Elland Road after a challenge by defender Conor Shaughnessy.
The Brazilian is on loan at Fulham from Chelsea for a second season, having played 33 times for the club last term.
"We have probably lost Lucas Piazon because we believe it is a bad injury," Fulham manager Slavisa Jokanovic told BBC Radio London.
Speaking in a press conference after the match, Jokanovic continued: "It's not a question about complaining - it can be bad luck or a tackle outside of the normal - but I'm really disappointed with what I heard from one of the referees who explained: 'This is English football and this is completely legal'.
"I am a 48-year-old man and I've never heard an ugly answer like I heard in this stadium."
When asked to clarify who he was referring to, Jokanovic said: "I don't see so well. I'm a little bit disappointed that I hear around me something so ugly."
It is the second time Piazon has suffered a serious injury this year, having broken his jaw while playing for Fulham against Cardiff in February.