The move is retroactive to Saturday. To take Chacin's place on the roster, the team recalled right-hander Rob Scahill from Triple-A Colorado Springs on Sunday.
Chacin was cruising Friday against Arizona when he stretched and grimaced after his 99th pitch. He grabbed his side after his 100th pitch and then left the contest.
Initially, the Rockies thought it might be a strained left oblique, but after reevaluating Chacin the next day the team said the injury centered on his back.
The hard-throwing righty is 3-0 with a 1.46 ERA this season. He was the Rockies' starter on opening day.
Scahill has made five relief appearances for the Sky Sox, going 2-0 with a 4.26 ERA.