Given Minnesota's bullpen depth, the Twins opted to trade the hard-throwing Welker for Johnson, who was mostly used as a starter at Triple-A Indianapolis last season.
"We were looking for starting pitching," Twins general manager Terry Ryan said. "It doesn't mean we didn't like what we had in Welker."
Johnson, 29, posted a 2.39 ERA in 26 outings (21 starts) at Triple-A with 94 strikeouts and 43 walks in 135 2/3 innings. He also made four appearances with the Pirates, including one start, posting a 6.10 ERA with nine strikeouts and four walks in 10 1/3 innings. The 6-foot-4 left-hander's lone start came against the Cardinals on Sept. 1, and he lasted just two innings, giving up five runs on seven hits.
Ryan confirmed the Twins' plan to keep Johnson as a starter next season, saying he believes the lefty has the arsenal to remain a starting pitcher. In limited time in the Majors, Johnson's fastball averaged 92.1 mph, and he mixed in a cutter, curveball and changeup, according to data at Fangraphs.com.
Johnson isn't overpowering, as evidenced by his 6.2 strikeouts per nine innings in his eight seasons in the Minors. The former first-round Draft pick by the Red Sox in 2006 also spent most of 2011 pitching for the independent Kansas City T-Bones of the American Association.
"He's certainly capable," Ryan said. "He has the pitches. He had a nice year at Triple-A. He didn't pitch much in the Majors, but he did a decent job at Triple-A."
Welker, meanwhile, was used exclusively as a reliever last season, posting a 3.57 ERA with 65 strikeouts and 31 walks in 63 innings at Triple-A Indianapolis. The 27-year-old also made two scoreless appearances for the Pirates in late June.
Rhett Bollinger is a reporter for MLB.com. Read his blog, Bollinger Beat, and follow him on Twitter @RhettBollinger. This story was not subject to the approval of Major League Baseball or its clubs.