The Ecuador international was rushed to hospital wearing an oxygen mask after catching his studs in the turf and suffering a dislocated and fractured left ankle.
Alarm call: players show their immediate concern as Valencia is flat out
Sir Alex Ferguson paid the price for making 10 changes as Rangers held out for a goalless draw. But players could barely look as Valencia writhed in agony around the hour mark.
Ferguson said: 'It looks like a dislocation and fracture. That's a bad one. It will keep him out for a long time. He seemed to catch his toe in the turf and we knew straight away it was a bad one from the way the Rangers player started to wave.'
Stuck in the mud: Valencia faces a lengthy spell on the sidelines
Valencia had only just returned to the side after recovering from a knee problem and faces a lengthy and arduous period of rehabilitation that will follow an operation scheduled in Manchester for today.
Wayne Rooney and Rio Ferdinand both returned to Ferguson's team but the United boss made eight other changes from the side held 3-3 at Everton in the Barclays Premier League on Saturday.
Nightmare: Wayne Rooney can't bear to watch as Valencia is treated
Ferguson added: 'People will talk about the team I picked but I thinkI played a strong team, a very strong team. 'Many of them played in theCommunity Shield against Chelsea. We should have won.
'Rangersjust went back to their penalty box and stayed there and they were successful. Give them credit for that.'
Rangers manager and former United assistant Walter Smith said: 'I'm proud of my players. We have to find a way to try and nullify the opposition.'
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