The righty thrived for most of his tenure with the St. Louis Cardinals, which started when he was acquired from the Marlins on July 31, 2012.
Mujica was elevated to the closer's role by St. Louis last season, when he converted 37 of his 41 save opportunities and posted a 2.78 ERA.
However, Mujica had a brutal September, posting an 11.05 ERA over 10 outings and losing the closer's role before the playoffs began.
As the Cardinals advanced to the World Series, eventually losing to the Red Sox in six games, Mujica had fallen out of favor and was used just twice in October.
Mujica joins a Boston bullpen that has a returning closer in Koji Uehara. The Red Sox also have Junichi Tazawa, Craig Breslow and Brandon Workman returning for high-leverage situations, meaning Mujica will join a fairly deep bullpen.
The 29-year-old Mujica has pitched in 381 Major League games for the Indians, Padres, Marlins and Cardinals, going 19-19 with a 3.75 ERA.
When Mujica arrived at Spring Training, he was expected to be St. Louis' seventh-inning righty. When Jason Motte was injured, plans had to change. Mitchell Boggs was thrown into the closer's role, but he struggled. The Cardinals turned to Mujica on April 18 for a save situation, and he converted it. The job was Mujica's the rest of the way, until his late-season collapse.
There was speculation that fatigue and injuries had more to do with Mujica's struggles than anything else. He converted his first 21 save opportunities before blowing his first on July 4.
In mid-August, Mujica began having some pain in his upper back/shoulder area. Mujica kept pitching, but it caught up to him in September. He was eventually replaced by young flamethrower Trevor Rosenthal in the closer's role.
Ian Browne is a reporter for MLB.com. Read his blog, Brownie Points, and follow him on Twitter @IanMBrowne. This story was not subject to the approval of Major League Baseball or its clubs.