To make room on the 40-man roster, the Twins designated Logan Darnell for assignment. He too will turn 27 before spring training begins. Unlike Strong, Darnell has been called up twice, once in 2014 when he fared ill, and again last September, when he fell ill and was shut down without ever pitching. Both men have options remaining.
Just going off their minor league numbers, Darnell doesn't appear to be a worse bullpen prospect than Strong, But Strong, it appears, has better raw stuff. Baseball America last spring rated Strong as the Brewers' 29th best prospect (albeit in a rather poorly regarded farm system); Darnell was not among the Twins 30 best in BA's rankings.
From the Baseball America Prospect Handbook 2015's entry on Strong:
His fastball command was much improved, helping set up his curveball and changeup while holding his walks down considerably (2.7 per nine innings) from previous seasons. Strong throws his fastball with confidence in the 88-92 mph range with good movement. His curveball is especially tough on lefthanders and his changeup is average and sometimes a tick above.
That 2014 command apparently faded last year, and really deteriorated when he moved up to Triple A Colorado Springs. My take: If Strong comes into camp throwing strikes, he's got a legitimate chance to win a bullpen job; if not, somebody else will be the primary setup lefty.
My mild apologies for skipping the past two days here while I dealt with an illness. It's been a long time since I failed to make my daily post, much less two, but there was nothing obvious I wanted to comment on and I didn't feel like wracking my brains to come up with something.