Irving was out for the third straight game with a bruised knee. Jack was expected to make his third start in a row for Irving, but he was a late scratch with back spasms. The Cavs said Jack couldn't get his back loose while warming up before the game.
With Irving and Jack out, rookie Matthew Dellavedova made his fourth start.
Irving injured his left knee on Dec. 31 when he fell awkwardly while attempting a layup in the third quarter of a loss to the Pacers. Irving said he felt "a pop" in his knee and had to be helped off the floor before returning in the fourth quarter. An MRI taken the following day showed no structural damage, and the Cavs say he is day to day.
Cavs coach Mike Brown does not know when he'll have Irving, his leading scorer, back on the floor.
Irving is averaging 22.2 points and 6.1 assists. The 21-year-old, who missed 38 games in his first two NBA seasons with a variety of injuries, played in 31 straight games before missing Thursday's game against Orlando.
Cleveland had lost seven of eight entering Sunday's game.