Minnesota's Gibson scratched from Wednesday start
Gibson, 8-8 with a 4.19 ERA, will be replaced by Anthony Swarzak.
Manager Ron Gardenhire says team doctors told him Gibson's back felt tight after throwing in the bullpen and then it locked up.
Swarzak is 1-0 with 4.34 in 30 relief appearances this season. The right-hander has 28 career starts but hasn't started a game since 2012.