The White Sox made the moves before Wednesday night's game against St. Louis.
Jennings, a left-hander, has been on the DL since June 7, retroactive to June 5, with inflammation on the left side of his neck. He tossed six innings in three relief appearances during a rehab stint with Charlotte.
The White Sox acquired Jennings during the offseason from the Marlins. He has a 7.83 ERA in 22 appearances covering 23 innings.
Guerra made three relief appearances in June and allowed three runs in four innings. He was 2-6 with a 2.72 ERA with 66 strikeouts in 12 games, eight of them starts, with Double-A Birmingham and Charlotte.