His apology came before a pregame ceremony in which the team honored its 2004 team that won Boston's first World Series championship since 1918.
''I realize that I behaved bad in Boston,'' Ramirez said. ''The fans were great. I also played great when I was here, but I realized I behaved bad. I apologize for that. I'm a new man.''
He said he has been attending church for almost a year.
During seven-plus seasons with Boston, Ramirez hit .312 with 274 homers and 868 RBIs before being traded to the Los Angeles Dodgers in 2008. While with Boston, he often jogged to first base and after fly balls, had a physical altercation with traveling secretary Jack McCormick and exhibited quirky behavior that led to the phrase, ''Manny being Manny.''
Ramirez was the last of more than 30 members of the 2004 team introduced before Boston's game against the Atlanta Braves. He was the MVP of that World Series sweep over St. Louis.
He threw the ceremonial first pitch, but Johnny Damon cut it off, rolled to the ground and relayed the ball to catcher Jason Varitek behind the plate. It mimicked a play in 2004 when Damon fielded a ball off the center-field wall and threw toward third base. But Ramirez inexplicably dove and cut it off rather than let it go through, allowing Baltimore's David Newhan to get an inside-the-park homer.
Ramirez received a warm reception as he entered the field through a door on the left field scoreboard in the Green Monster. He occasionally would use the door to go into the cramped area behind the scoreboard during pitching changes.
On Wednesday night, the crowd chanted ''Manny, Manny,'' when he was introduced and walked toward the infield.
Current designated hitter David Ortiz is the only Red Sox player remaining from the 2004 team.
''Now that I see David and the rest of the guys, I wish I could have done better,'' Ramirez said. ''I wish it turned out better than it did.''
Now he hopes to make a good impression on players in the Cubs' farm system.
''I'm looking forward to that,'' Ramirez said. ''I can go over there and meet those kids and teach them what to do in the field and what not to do off the field. It's going to be a blast. We're going to have fun out there.''
Other players from the 2004 team who received loud ovations where pitchers Pedro Martinez and Curt Schilling and outfielder Kevin Millar. Schilling, who has skipped other Fenway Park events this season while being treated for cancer, was accompanied by his son Gehrig as they walked in together from left field.
Keith Foulke, who was on the mound for the final out of the 2004 World Series, carried the championship trophy as he made his way to the infield.
''You work your entire career and you always dream of being on top for at least once,'' Foulke said before Wednesday's game. ''We're honored to be a part of something that, unfortunately, had been set up by a lot of failure. We're proud to be a part of it, I'll tell you that.''
Ramirez appeared with a haircut in which the sides of his head were shaved, leaving hair only on top.
''It's Manny,'' Foulke said, with a laugh.