Mejia was traded to the Twins on Thursday in a move that sent Eduardo Nunez to the Giants.
The 23-year-old lefty was one of the Giants' top overall prospects and their third-best pitching prospect prior to being sent to Minnesota. He struggled a bit after being promoted to Triple-A Sacramento, posting a 4.20 ERA and a 1.32 WHIP over seven starts, but that's not entirely uncommon in the hitter-friendly Pacific Coast League. Mejia flashed dominant stuff at Double-A earlier this season with a 1.94 ERA, 0.98 WHIP, 58 strikeouts, and just 16 walks. He will be assigned to the Twins' Triple-A affiliate at Rochester.