Paxton (forearm) was placed on the 10-day injured list Wednesday.
Paxton was initially diagnosed with a left forearm strain after leaving Tuesday's loss to the White Sox, but he'll undergo an MRI on Wednesday to determine the extent of the issue. Ljay Newsome was recalled in a corresponding move, but the Mariners are expected to fill Paxton's spot in the six-man rotation with long reliever Nick Margevicius.