Tillman, who started the season on the disabled list, entered 2017 having started at least 30 games in each of the past four seasons. He went 16-6 with a 3.77 ERA last season for the O's.
Showalter noted a move to the bullpen has helped other pitchers out in the past.
"It's done good things for Ubaldo [Jiménez] and some other people in the past," Showalter said. "Did some real good things for Zach Britton on the other end of it. But I know Chris, talking to him, physically he feels great, he feels good, and that's been frustrating for all of us, so we're trying to figure out some way to try to get him back."
Tillman allowed seven runs -- five earned -- on six hits in two innings during a 7-5 loss to the Tigers on Thursday.
"Like they've been doing all year, they made some more adjustments to see if he feels better," Showalter said. "It's tough for him to have a workday and do things that he can work on, but Ubaldo seemed to find a way to make it productive. I know Chris will, too, because those guys are going to need to pitch, and they're going to need to do well."
Showalter will utilize a five-man rotation without Tillman after using six starters following the trade for right-hander Jeremy Hellickson.
• Outfielder Mark Trumbo (right rib cage strain) is on track to rejoin the Orioles on Wednesday, Showalter said. Trumbo landed on the 10-day DL on Monday, retroactive to Sunday. Rather than going on a rehab assignment, the slugger will travel with the team on its 10-game road trip that opens Monday against the Angels.
• Catcher Caleb Joseph (right wrist) was available off the bench for Saturday's game against Detroit. Joseph was hit by a pitch Friday, but X-rays were negative.
Ben Standig is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.