Martinez agreed to a $1 million, one-year contract with the Phillies on Wednesday.
The team put the three-time Cy Young winner on the 15-day disabled list with a shoulder strain. Phillies general manager Ruben Amaro Jr. has confidence Martinez will be ready to pitch in two to three weeks.
Martinez went 5-6 with a 5.61 ERA for the New York Mets last year. He pitched in the World Baseball Classic before this season.
"The last few years have been horrible," Martinez said. "I haven't felt the way I wanted to feel. I wasn't the same Pedro you were accustomed to seeing."
Martinez can earn up to $1.5 million in incentives. The eight-time All-Star will need to make at least a few starts in the minors before he's ready to join Philadelphia's depleted rotation.
The 37-year-old Martinez was in good spirits, cracking jokes and telling stories before holding up his No. 45 jersey. He topped 91 mph in workouts scouted by the Phillies and believes his injury problems are in the past.
"I was stupid for pitching not in the regular health that you should have been," Martinez said.
The defending World Series champion Phillies have been hit by injuries to their starting rotation. Brett Myers had hip surgery in June. Antonio Bastardo, who replaced Myers, landed on the disabled list after making five starts. The NL East leaders already have used nine starters through 86 games. They used only seven starters last season.
"Of course there's some risk here," Amaro said. "But we'd rather risk it on someone of his caliber and who knows how to win."
Martinez is 214-99 with a 2.91 ERA in 17 seasons with the Los Angeles Dodgers, Montreal, Boston and the Mets. He helped the Red Sox win the World Series in 2004.