Injuries hurt Casilla's chances of joining the Orioles earlier in the season. But the utility man's versatility and well-regarded defense is something the team values.
"If he had been healthy, he would have been here a while ago," manager Buck Showalter said. "He's a guy that everyone knows, and has been a contributor in the past."
Casilla, who has been rehabbing an injured hamstring and missed time earlier in the season after undergoing surgery on his right hand, hit .264/.315/.320 in 56 Triple-A games this season. He stole nine bases in 11 attempts, and can play both second and shortstop.
The eight-year veteran played 62 games with the Orioles last season, hitting .214/.268/.295 with nine stolen bases.
For Casilla and players fighting for inclusion on the Orioles' postseason roster, Baltimore's first-round opponent will be a critical factor in deciding who the team carries. Showalter does, however, intend to deploy a larger bench for the playoffs.
"I still haven't made up my mind on whether we are going to go 10 or 11 pitchers in the postseason," said Showalter. "[Our] Game 3 and 4 starters are in the bullpen Game 1 and 2."
Guilmet, meanwhile, had a 5.23 ERA over 10 appearances with the Orioles this season.
Casilla went 0-for-4 in Saturday's 4-2 loss to the Blue Jays.
Chris Toman is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.