CLEVELAND -- Nick Sandlin, a key member of the Guardians' league-best bullpen, was placed on the 15-day injured list, retroactive to Monday, with low back inflammation on Wednesday.
Cleveland is in the midst of a three-game home series against Seattle.
The right-hander has ranked among the American League leaders in appearances all season, posting a 5-0 record and 3.49 ERA in 33 games. He experienced discomfort during Cleveland's recent 10-day trip, prompting him to be evaluated Wednesday.
Guardians manager Stephen Vogt said he anticipates having Sandlin back when first eligible, but "the injury is preventing him from being himself right now." He allowed three runs without recording an out Sunday in the Cleveland's 7-6 loss in Toronto.
Cleveland's relievers lead the majors with a 2.41 ERA, 20-7 record, 60 holds and a .194 opposing batting average. The Guardians are the only bullpen with a sub-3.15 ERA and the lone group holding foes below a .202 average.
Emmanuel Clase has an AL-leading 21 saves, while Sandlin, Scott Barlow, Cade Smith, Hunter Gaddis and Tim Herrin also have pitched in at least 32 of the first 70 games.
Right-hander Darren McCaughan was recalled from Triple-A Columbus to replace Sandlin on the active roster. He gave up eight runs in 4⅔ innings with Miami before being dealt to Cleveland on May 9.