The move was made Monday before Minnesota's game at Atlanta. The 26-year-old Thielbar is a native of Randolph, Minn., about 40 miles south of Minneapolis.
Two seasons ago, Thielbar became the first player from the independent league St. Paul Saints to have his contract purchased by the Twins. Thielbar had a 3.76 ERA with 34 strikeouts in 26 1-3 innings in relief this season for Rochester.
Hernandez had a 5.85 ERA. In four starts this month, he allowed 30 hits, 17 runs and four homers over 17 innings.