"He's got to get surgery and he'll be out two or three months," Pastner said.
Pastner said that a flap of cartilage in Thomas' ankle came loose during a practice last Friday. An X-ray later that day showed no bone damage, but an MRI on Monday evening revealed the misplaced flap of cartilage.
Thomas is third on the team in scoring at 9.7 points per game and second on the team in minutes at 26.3 per game. He also averages 3.6 rebounds.
Thomas missed the Tigers'89-55 win at Houston last Saturday.
Pastner said he, Thomas' family, and team doctors came to a conclusion after the MRI that surgery was the best route to take.
"He could play on it, but we just felt like for the long-term, for Adonis, (surgery) is the right thing for his career," Pastner said. "He'd be in pain the entire next three months. The doctors felt the best scenario was to get the surgery done."
Thomas' injury leaves a huge hole in an already-thin frontcourt. The Tigers will likely rely upon senior forward Wesley Witherspoon to fill the void.
The Tigers are also still without senior guard Charles Carmouche, who has missed a multitude of games with tendonitis. Carmouche, who has not played since a 69-51 win against Tennessee on Jan. 4, will not play against UCF on Wednesday.
At any rate, Pastner said his team has no choice but to rally around one another and move forward.
"We've lost two starters," he said. "Guys are gonna have to step up there's no other option."