Manchester United manager Alex Ferguson confirmed Alan Smith broke his leg and dislocated an ankle towards the end of the 1-0 FA Cup defeat against Liverpool.
The England international came on as a substitute for Darren Fletcher with 15 minutes remaining of the fifth-round tie at Anfield.
And, as he attempted to block John Arne Riise’s shot following Dietmar Hamann’s free-kick, his left leg crumpled underneath him.
Smith, whose hopes of a place in England’s World Cup squad appear to be in jeopardy, needed oxygen as he was carried off on a stretcher, and Ferguson reported it was “one of the worst” injuries he had witnessed.
“He has broken his leg and dislocated the ankle joint,” the United boss told MUTV. “It is a bad one. He has gone to hospital, it looks very long term – it is one of the worst I have seen. I am sure he will be back.”