The injury occurred when Florida State guard Aaron Thomas stole the ball from Hairston at the top of the key with 4:15 to play.
Hairston's explanation, according to UNC head coach Roy Williams, described the ball being jammed between his two fingers. UNC athletic trainer Chris Hirth spent several minutes wiping up blood off the Greensboro Coliseum court following the incident.
"We're extremely concerned right now," Williams told reporters during his postgame press conference. "His hand is torn up… We have no idea, I have no idea, won't have any idea. I've told you everything I know. It doesn't look good."
Hairston was unavailable to the media following the game.
The Greensboro, N.C. native scored a game-high 21 points, including connecting on 5-of-6 three-point attempts, against the Seminoles. He had started nine straight games at the 4-spot in UNC's small lineup.
When asked about the difficulty in preparing for No. 7 seed Maryland's variety of lineups in semifinal action on Saturday, Williams replied: "The most complicated thing we have right now is what's going to happen to P.J. And I have no earthly idea."
If Hairston is unable to play against Maryland, Williams may be forced to revert to his earlier lineup.
"The conventional wisdom would say you have to go back big," Williams said, citing the coaching challenges involved such as teaching wing players the out of bounds plays from the 4-spot.
The 10th-year UNC head coach referenced John Henson's wrist injury in the ACC Tournament quarterfinals last March in Atlanta, which opened the door for James Michael McAdoo to emerge as a legitimate post option for the Tar Heels.
"If we can't have P.J., somebody's got to step up and play great," Williams said.