The Wales international was carried off on a stretcher during Forest’s 3-1 win over Brighton on Wednesday.
Williams, 22, collided with team-mate Brennan Johnson as the pair tried to reach a chipped cross.
“He was playing with confidence, so it’s a blow for him and a blow for us,” said manager Steve Cooper.
“Everybody saw the collision on the pitch and when someone gets stretchered off, you always fear the worst.
“It’s a difficult situation for him, he’s recently got back into the team and he’s been playing well.”
Writing on Twitter, Williams said: “I am gutted to say my season’s over for sustaining a broken jaw.
“I will do everything I can to help my team-mates bring home those points every game.”
Cooper’s side are 17th in the Premier League, one point off the relegation zone with five games left to play.
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