Long was recalled from Triple-A Sacramento on Tuesday.
The 26-year-old was sent to Sacramento on Friday but will rejoin the Giants with Alex Cobb (neck) headed to the injured list. Long has started three games for San Francisco this season but has covered only 12.1 frames in 10 appearances, and he's not fully stretched out as a starter. The team has yet to announce who will fill Cobb's spot in the rotation, but Long is likely to be considered.