The 22-year-old, who was taken off during the Grecians 2-2 draw with Barnet, underwent an operation on the ankle on Thursday which means the first team regular`s campaign is effectively over.
"It looks like Liam is going to be out for the season," City boss Paul Tisdale said. "It looks like he will be out of training for three months which means four for playing.
"So to all intents and purposes it puts him out for the rest of the season which is a real shame, a real loss for us.
"I am sure he is feeling very frustrated and disappointed about it."
The City youth product opened his account for the season in the 1-1 draw with Plymouth in December and was one of the Grecians' top performers in their next match against Barnet before sustaining the injury nine minutes after the interval .
"He seemed to be, in that Barnet game, looking as purposeful and as determined as I've seen him for a long time," Tisdale said.
"We've been encouraging him to find some of the form he's showed on many occasions for the club in that midfield role and he was doing that against Barnet.
"He had a couple of good works, so it is a real shame for us."