Gentry's outright clears a 40-man roster spot and that, coupled with having Castillo, Francisco Pena and Caleb Joseph on the roster, is an indication of more moves coming.
"Tough [move], he's been in what, 33 of 36 games?" Orioles manager Buck Showalter said of Gentry. "He's been a real contributor and will be again. We'll probably make another adjustment to the roster here shortly."
Castillo, who had been sidelined with tendinitis in his right shoulder, was in the starting lineup on Tuesday against the Tigers. The 30-year-old backstop was hitting .314 with one home run and six RBIs in 17 games before the injury.
Gentry, 33, batted .162 with one home run, four RBIs and three stolen bases in 33 games for Baltimore.
Brittany Ghiroli has covered the Orioles for MLB.com since 2010. Read her blog, Britt's Bird Watch, follow her on Facebook and Twitter @britt_ghiroli, and listen to her podcast. This story was not subject to the approval of Major League Baseball or its clubs.