Manchester United manager Sir Alex Ferguson fears Antonio Valencia will be unavailable for the remainder of the season after suffering a fracture or dislocation of his ankle.
The Ecuador winger sustained the injury in Tuesday's 0-0 draw with British rivals Rangers in their opening match of Group C in this season's UEFA Champions League.
Valencia slumped to the ground in the second half of the match after apparently catching his foot in the Old Trafford turf, with only blameless Rangers defender Kirk Broadfoot in the vicinity.
The 25-year-old was stretchered from the field while wearing an oxygen mask and Ferguson, who controversially made 10 changes to the team which drew at Everton at the weekend, has confirmed that surgery is scheduled for Wednesday.
"It is a dislocation or a fracture and he gets operated on tomorrow morning," said the United boss.
"It was an innocuous challenge but when you saw the Rangers player waving, you knew it was a bad one.
"It is a bad blow for the boy. It is similar to the injury Alan Smith got and you are talking about the rest of the season."
Ferguson defended his team selection against the Scottish champions, which was presumably decided upon with an eye on Sunday's arrival of Liverpool in the Premier League.
Surprisingly, only Scotland international Darren Fletcher retained his place from the draw at Everton, although Ferguson was able to recall Wayne Rooney and Rio Ferdinand.
"The criticism will be why didn't I play a stronger team," he said.
"I think I played a very strong team. People forget eight of them played against Chelsea in the Charity (Community) Shield.
"It's a terrific squad of players we have and there was no problem playing them.
"They should have won that game, but give credit to Rangers, their system is very difficult to break down."
Ferguson added: "I think we played with great intensity and good commitment. But Rangers just went back to the penalty box and it was successful.
"We said they had two draws away from home and lost 1-0 in Seville last season so they have a successful format."
Was Sir Alex Ferguson right to make 10 changes to his team against Rangers?