"I think it was a mechanical adjustment: My arm wasn't really coming through at the right time," Pettibone said of his early struggles. "I was able to iron that out and it is something I have worked on in the last couple of bullpens."
Pettibone allowed all three runs in the second inning, when he threw 32 pitches. Double-A Reading scored on an RBI single by Edgar Duran, a run-scoring groundout by Albert Cartwright, and an RBI double by Collier.
The Phillies need a fifth starter on April 13 against the Miami Marlins at Citizens Bank Park. Manager Ryne Sandberg has said that Pettibone is among the candidates.
IronPigs manager Dave Brundage said Pettibone was still shaking off some rust.
"There were times he didn't have his great command that he is capable of having, but he said he felt good," Brundage said. "I thought the ball came out of his hand the last couple of innings a little bit better."
Pettibone missed the final two months of last season with a strained right shoulder. He experienced shoulder soreness early in spring training and is still rounding into shape.
In his previous outing, Pettibone threw two shutout innings in the Phillies' final spring-training game in Florida, a 3-0 loss to Toronto.
Pettibone began last season at Lehigh Valley but was recalled by the Phillies on April 22. In 18 starts, he was 5-4 with a 4.04 ERA.