Hairston, the Tar Heels' third-leading scorer, did not practice Monday, and the school announced he would not travel to Bloomington, Ind.
"We don't think ... it's anything serious, long term," Tar Heels coach Roy Williams said Monday during his radio show. "But he sprained his knee, didn't feel like he could do anything in practice today. We're coming so close to the end of the semester -- if you can't play, stay home and study."
A team spokesman said earlier Monday that Hairston, who is averaging 10.8 points and 4.2 rebounds while shooting 35.3 percent on 3-pointers, has a mild left knee sprain.
Williams said during his news conference earlier Monday that Hairston's absence would limit the 14th-ranked Tar Heels' opportunities to go to a smaller lineup.
"He's the only guy (at the wing) that's worked at the 4," Williams said. "Nobody else has."
With so much youth in the post, North Carolina has been counting on its experienced outside shooters and has found some success with the 6-foot-5 Hairston playing occasionally at power forward.
"It's a difficult thing for us, because the one positive that we got out of the trip to Hawaii (in the Maui Invitational) was the fact that we could go small and cause some people some problems, and that was with P.J. in the game as a 4-man,'" Williams said. "He was a difficult matchup for guys, because he's a very strong youngster, he's a very good defensive player, so he could guard somebody in the power forward slot."
Teammate Dexter Strickland said Hairston's injury occurred during dribbling drills. And although the senior said his teammate "is a little dramatic with his injuries," Strickland doesn't anticipate Hairston playing against the Hoosiers.
"I'll give him about two days, and he'll probably be good -- back out on the floor, running again," Strickland said.
North Carolina said Hairston's status for Saturday's game against UAB is unknown.