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General manager Neal Huntington admitted it was a difficult call to cut ties with Locke, especially as the Bucs search for pitching help this offseason.
"We are talking about needing starting pitching, and Jeff has been at various points -- probably more than he gets credit for -- a solid Major League starting pitcher," Huntington said. "Unfortunately, it's not the first guy we've had to make a tough decision on as they go through the arbitration process. Unfortunately, he's probably not going to be the last."
Acquired along with Charlie Morton and Gorkys Hernandez in the June 2009 trade that sent Nate McLouth to the Braves, Locke leaves the Pirates with a career 35-38 record and 4.41 ERA over parts of six seasons. A serviceable back-end starter from 2013-15, Locke changed his mechanics last season and posted a 5.44 ERA in 127 1/3 innings, pitching his way out of Pittsburgh's rotation.
The high point of Locke's tenure in Pittsburgh came in 2013, when a strong first half earned him a spot on the National League All-Star team. He flashed his ability even this year, throwing a three-hit shutout in Miami on May 30 and beating Madison Bumgarner at PNC Park on June 20.
But Locke was plagued by inconsistency and ultimately bumped to the bullpen, where he finished the season as a mop-up reliever. Still, he wound up leading the Pirates' battered staff in innings pitched.
"We certainly appreciate all that Jeff has done, the time he's put in. The inconsistency wasn't from a lack of effort or caring. He certainly worked and he certainly cared. For whatever reason, we just weren't able to help him put it together the way we have with many others," Huntington said.
At the end of the season, Locke spoke with MLB.com about his uncertain future. While he hoped to remain in Pittsburgh, he also acknowledged he'd prefer to start if given the opportunity.
"There's availability in other areas," Locke said. "That's what the great opportunity of this game is. If you have an arm, you have a pair of spikes, you have a track record, somebody will give you a shot."
Bonilla, 26, pitched for the Dodgers' Double-A and Triple-A affiliates this past season, finishing 5-7 with a 3.97 ERA and 118 strikeouts in 111 innings over 31 appearances, including 13 starts. It is unclear whether he will be a starter or reliever for the Pirates.
Bonilla, who missed the 2015 season following Tommy John surgery, briefly reached the Majors with the Rangers in 2014. He went 3-0 with a 3.05 ERA in 20 2/3 innings over five appearances. As a prospect, he was selected to play in the SiriusXM All-Star Futures Game in 2012.
"It's a guy we've liked the last couple years," Huntington said. "We felt like it was a good opportunity to add what we believe to be a prospect pitcher."
Bonilla is 32-27 with a 3.33 ERA and 10.1 strikeouts per nine innings over 201 appearances in the Minors. Bonilla will report to Spring Training with a shot to make the Bucs' big league roster as either a starter or reliever, but he has Minor League options remaining.