"I feel good," Pettitte said. "I feel better today than I did [Sunday]. We haven't even talked about me missing a start."
Pettitte first appeared to strain his back in the fourth inning Friday against the Cleveland Indians, but remained in the game. He took a shutout into the sixth inning, but was removed after allowing a single and a walk.
Pettitte normally throws two bullpen sessions between starts, but didn't throw off the mound Sunday. He threw 25 pitches Monday.
The 37-year-old left-hander is 5-1 with a 4.10 ERA.
He threw 42 pitches in three scoreless inning in Sunday's 5-4 loss to the Indians and said after the game he feels he's ready to rejoin the rotation. Wang also pitched two perfect relief innings in Texas on May 27.
Wang, who won 18 games in 2006 and '07, began the season in the rotation, but was 0-3 with a 34.50 ERA in three starts before going on the DL.
"I would expect [Wang] him to be frustrated, but I haven't seen it," Girardi said. "He's done everything we've asked. We've talked all along that we believe he's a starter. At some point we believe he's going to be in this rotation."