Detroit sent left-hander Casey Crosby and outfielder Matt Young to Toledo on Friday night to create space on the roster.
Fister injured his left side during his first start of the season against Boston in April and went back to the DL on May 29 with the same injury. He is 0-3 with a 3.15 ERA after going 8-1 with a 1.79 ERA last season with the Tigers.
Weber pitched in two games for the Tigers earlier this season and gave up four runs in four innings.
Crosby is 1-1 with a 9.49 ERA as a rookie Young had one hit in 10 at-bats in five games with the Tigers.