Flaherty's last game was on Sunday, when he went through the tough adjustment of trying to catch the baffling knuckleball of Tim Wakefield.
A big qualification for whoever wins the backup job behind starting catcher Jason Varitek will be the ability to catch Wakefield, something Mirabelli -- now on the Padres -- handled masterfully during his time with Boston.
However, Red Sox manager Terry Francona didn't think the prospect of having to handle the knuckleball on a regular basis had any bearing on Flaherty's decision to end his career.
"I think he's been thinking about this since the beginning of camp," said Francona. "He wasn't sure he was able to do what he needed to do for the Red Sox like he has in the past for other teams. I think he has had mixed emotions all spring, and only because it was the Red Sox, that was probably the only reason he even came to Spring Training."
The competition for the backup spot will now be waged between Josh Bard and non-roster invitee Ken Huckaby.
Huckaby has been sidelined the past couple of days with soreness in his left knee. He had the knee operated on in the offseason.
With Varitek playing for the United States in the World Baseball Classic, Bard will get the majority of starts the next few days.
"Hopefully, Huckaby will come back soon, but we're not going to catch Bard every day," said Francona. "I had a talk with Huckaby today, and he's dying to get out here and he's champing at the bit, and what he's going to do is set himself back, and I don't want him to do that. That doesn't make sense. We'll catch Bard every other day. We'll keep bringing some kids up here to catch, especially the ones who can handle it defensively."
Flaherty was considered in many circles to be the front-runner to win the backup catching job before he decided to retire.
"John is a true professional and class act with much to be proud of during his distinguished career," said Epstein. "We'll miss his presence and contributions, but we respect and support his decision. We're proud that John began and ended his career in a Red Sox uniform, and we wish him the best wherever life leads him."
The 38-year-old Flaherty, who signed with the Red Sox on Dec. 23, hit .252 in his career with 80 home runs and 395 RBIs. He played in 1,047 games with the Red Sox (1992-93), Tigers ('94-96), Padres ('96-97), Devil Rays (1998-02) and Yankees ('03-05).
"Hopefully, it's something he's at peace with," said Red Sox ace Curt Schilling. "He's a classy guy. I wish him the best. It's a tough situation, I'm guessing, because from a pitcher's standpoint, you've got to be a backup catcher that has to catch Timmy. That represents a whole new set of challenges for whoever that guy is."
Francona was a coach on the Tigers staff in 1996 when he first got to know Flaherty.
"I'm actually kind of happy for him," Francona said. "He's kind of going out on his own terms. And he's had a really good career. I saw this kid in '96 when he was a kid. We talked about that a little bit. I think he's comfortable with what he's doing."