Thielbar (hamstring) will begin a rehab assignment with Triple-A St. Paul on Tuesday, Aaron Gleeman of The Athletic reports.
The Twins' bullpen has been hit hard by injuries, but they should get one of their key relievers in Thielbar back soon. He posted a 3.23 ERA, 0.95 ERA and 36:6 K:BB over 30.2 innings for Minnesota last season.