Dimitar Berbatov may have to undergo exploratory knee surgery, according to Manchester United manager Sir Alex Ferguson. The Bulgaria international was not involved on Saturday when the 10-man champions salvaged a 1-1 draw against in-form Birmingham at St. Andrews. Ferguson gave a debut to summer signing Mame Biram Diouf, who came on as a second-half substitute, and revealed former Tottenham striker Berbatov is struggling to shake off a knee complaint. He said: "Dimitar wanted to play with it but we felt we should try to get it (the knee) sorted and get him fit again. "He may have to have an exploratory operation."