Mirabelli, 35, is hitting just .182 (4-22) in 14 games this season, but is adept at catching knuckleballers like Boston's Tim Wakefield.
"We felt that there was probably no one on the planet better suited than Doug to do this," Red Sox manager Theo Epstein said. "We had a chance to get him at a reasonable cost, so that made sense to us."
Bard struggled to catch Wakefield, allowing 10 passed balls in seven starts.
Doug Mirabelli is back with Boston. (Frank Gunn/Canadian Press)
"We have no doubt he would have eventually figured it out with Wake," Epstein said. "It might've been three days from now, it might've been three months from now.
"We didn't necessarily have the luxury of the time. The single best solution [Mirabelli] was available now and we didn't want to wait."
"I think we all thought [Bard would] be able to do it in the long run," added Red Sox manager Terry Francona. "But we need to win now.
"What we're doing is trying to stay with some teams in our division. It was getting harder to do that."
Boston initially traded Mirabelli to San Diego for second baseman Mark Loretta last Dec. 7.
"A lot of thought went into it ahead of time [that is] the Mirabelli-Loretta deal," Epstein said. "It was sort of an educated guess that if you bring in enough guys, bring in guys with the right kind of defensive skills, hands and makeup, that they'll figure it out over time.
"It was a question of how long it would take. And if it didn't work out, would Mirabelli be available at the right cost?"
Mirabelli is a lifetime .240 hitter with 47 home runs, 165 runs batted in, 141 runs scored and only 21 fielding errors in 459 games over 11 MLB seasons for the San Francisco Giants, Texas Rangers, Boston Red Sox and Padres.
with files from Sports Network
