Guerra didn't allow a run in three games and 1 1/3 innings for the White Sox. He went on the disabled list with right shoulder inflammation on April 20, a move retroactive to April 13.
He had a 17.18 ERA in four games during an injury rehabilitation assigned at Triple-A Charlotte. The White Sox activated him from the disabled list Wednesday, then designated him for assignment.
Chicago remains responsible for his $937,500 salary. The team has 10 days to trade him, send him outright to the minors or put him on waivers.