Spending almost two months on the disabled list with shoulder tendinitis and bursitis, he went 6-9 with a 6.27 ERA, posting the fewest starts, wins and innings pitched and worst winning percentage of his career.
"For me, doing that was embarrassing," the 30-year-old right-hander said in a conference call.
Penny was 94-75 in nine years for the Marlins and Dodgers, going 16-4 with a 3.03 ERA in 2007 and finishing third in voting for the NL Cy Young Award. But he made just two starts and two relief appearances after going on the DL in mid-June.
"I was trying to push something I should have rested," he said. "Even when I was on the DL I was throwing. I was a little stubborn and tried to push myself."
His offseason conditioning is going well, he said.
"I've had no problems. I'm feeling pretty strong," Penny said. "I feel good right now, so hopefully it's cool."
The big right-hander joins a rotation that includes Josh Beckett, Daisuke Matsuzaka, Jon Lester and knuckleballer Tim Wakefield, with John Smoltz standing by to join them when he's ready. Penny was teammates with Beckett and Lowell, the Red Sox third baseman, when the Marlins won the 2003 World Series.
Lowell was acquired by the Red Sox, along with Beckett, after the third baseman dropped from a .293 average with 27 homers in 2004 to .236 and eight in '05.
"I've never been more excited for a season to start than this one," he said at the time. "I can't believe the six years prior to last year are a fluke and that's what I really am."
Lowell bounced back in Boston, averaging 20 homers and 100 RBIs over the next two seasons and winning the World Series MVP for the Red Sox in 2007; he struggled last year, playing in just 113 games because of a hip injury. Beckett came as advertised, going 20-7 in '07 and dominating in the postseason as the Red Sox won their second title in four years.
Now, Boston is hoping another ex-Marlin can manage a similar turnaround.
"That's not the Brad Penny last year. I know that, and hopefully the people in Boston, they know that," Penny said. "I'm real excited. I can't wait to get out there."
Penny said he consulted with Beckett and Lowell before signing with the Red Sox.
"All three of us were really close," Penny said. "That's probably the closest I've ever been to teammates."
Penny said he was lured to Boston by the chance to win, but only after visiting the ballpark on Thursday for his physical did he realize the total allure.
"I got back and told my parents, 'Man, just walking in there you could feel the history,' " he said.
Penny can earn an additional $3 million in performance bonuses: $500,000 each for 160 innings or 55 games as pitcher, 170-65 and 180-75; $500,000 for 190 innings or 35 games finished, and $1 million for 200 innings or 50 games finished.