Pettitte has been out since May 16 with an injury to his trapezius muscle (a.k.a. "traps"), which is the area between the shoulder and the middle of the neck.
Pettitte, 40, is 4-3 with a 3.83 ERA, 1.30 WHIP and 39 strikeouts in 49 1/3 innings this season. He was 5-4 with a 2.87 ERA in 12 starts last season after returning from retirement.
So the Yankees will have their top four in the rotation back, in CC Sabathia, Hiroki Kuroda, Pettitte and Phil Hughes. Either David Phelps or Vidal Nuno would then be the fifth starter. Monday would have been Phelps' next turn in the rotation, but he could easily be pushed back to Tuesday if need be.