In the second half at Old Trafford, Valencia was involved in a challenge with Rangers defender Kirk Broadfoot and required immediate attention. Television broadcasters decided against showing a reply - indicating the severity of the incident - and Valencia required lengthy treatment before being taken immediately to hospital.
After the match, United boss Sir Alex Ferguson said the club suspected Valencia had suffered either a dislocation or fracture of his left ankle and could be out of action for a lengthy period.
"It is a dislocation or a fracture and he gets operated on tomorrow morning [Wednesday],'' Ferguson said. "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."
Smith suffered a broken leg when playing for United against Liverpool in February 2006 and was sidelined for seven months, although team-mate Darren Fletcher - who was in the same side as Smith - is confident Valencia will still have a big future.
''It looked really bad,'' he said. ''Hopefully the scans show it is not as bad as we fear. After arriving from Wigan he had a great first season and was looking to push on this year. That is why the injury is a real blow.
''He has only just got back from the injury he suffered at the end of last season too. He is a strong character and a good lad. He has real determination and works hard. He will be back I am sure. He will have a long future at Manchester United.''
Ecuador national team doctor Patricio Maldonado told the Daily Mail: ''It is a double fracture to the tibia and fibula in his left ankle. He will be operated on.''
Hector Bohorquez, also a member of Ecuador's medical team, said the player would need psychological support. He added: ''This type injury has a huge impact on athletes who see how their bones have become detached. He must be continuously supported.''
Broadfoot spoke of his horror at witnessing the injury after the game and wished the Manchester United player a quick recovery.
''I'm not too sure what happened,'' he admitted. ''I just slid in and looked round and his bone was sticking out. We tried to get the physio on to help him because you don't like to see that.
''These things happen and hopefully he makes a fast recovery. I've never seen anything like that so close before. You see things like that on television and it's not good but, to see it there in front of you, it doesn't look nice and I just hope he recovers quickly.''