Horford suffered his knee injury in the first half of Sunday's 91-81 road win against the Detroit Pistons. He limped off the court and finished with 18 points and nine rebounds in playing 34 minutes.
Irving scored 16 points on 4-of-12 shooting, also playing 34 minutes against the Pistons. He came out of the game in the first quarter and was spotted removing his left shoe and stretching out his leg during a timeout, according to ESPN.
Irving missed one other game this season when he sat out against the Toronto Raptors on Nov. 12 after suffering a facial fracture the game before against the Charlotte Hornets. He wore a mask for 13 straight games before going without it in Sunday's game.
In 27 games this season, Irving is averaging 23.7 points and 4.8 assists.