Valencia last night had an operation on the ankle he fractured so horrifically in Tuesday's Champions League encounter with Rangers at Old Trafford.
Red Devils medical staff have assessed the Ecuador wide-man this morning and confirmed Valencia has also suffered ligament damage.
"The operation went well, a plate has been inserted to stabilise the fracture but there is obviously significant ligament damage that follows a dislocation," read a statement from United.
"He will be out for a good portion of the season."
In the immediate aftermath of the incident, United manager Sir Alex Ferguson ruled Valencia out of the remainder of the campaign and likened the injury to one suffered by Alan Smith in 2006.
The statement also denied claims attributed to the Ecuador team doctor about Valencia's mental state and insisted the former Wigan star was feeling positive about his recovery.
"Contrary to recent reports, there has been no official contact from the Ecuadorian national team," read the statement.
"Antonio is perfectly fine, very positive and very determined."
When Smith suffered his injury in a freak incident against Liverpool at Anfield, he was ruled out for seven months, claiming afterwards that his eventual return came too soon.
A similar timescale would have Valencia back in action by April, although by that stage Ferguson might not want to be pushing the winger anyway.
After an impressive first season at Old Trafford, Valencia had been hoping to make further improvement.
Instead, he must battle to recover from such an awful setback, with Darren Fletcher amongst those who feel he will be back.
"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."