Johnson will take the spot of lefty Brian Matusz, who was optioned to Triple-A Norfolk following Sunday's game. It will mark Johnson's first career Major League experience.
"Stevie is deserving, he's pitched well down there -- as have a lot of our guys down there -- and was a good fit for us," Showalter said. "I'm excited for him, it's a big moment for him. Hopefully I can get him in there."
"[I'm] definitely excited," Johnson said. "You work really hard to get here, so it's a dream come true. Hopefully I can do well and help the team."
The son of former Oriole and current MASN broadcaster Dave Johnson, the 24-year-old Johnson threw two innings on Saturday, and is 2-0 with a 0.53 ERA in five games since moving to the bullpen.
While Johnson solidifies the bullpen for now, the Orioles also made their starting rotation for the rest of the week a little more clear with the announcement that Tillman would join the club. Showalter said Tillman and Jake Arrieta will start Wednesday and Thursday, although it is unknown yet which pitcher will start when. Tillman's start on Monday with Norfolk was scratched, and it is likely he will take the hill Wednesday to keep close to his regular pitching schedule.
"If we do -- I'm not saying we are -- but the benefit would be to give Jake an extra day going into the break and to keep Tilly close to his start day, as opposed to too far removed," Showalter said.
Tillman is 8-8 with a 3.63 ERA and 92 strikeouts in 89 1/3 innings in 15 starts with Norfolk this season.
Friday's starter is still uncertain, but Zach Britton, who threw for Norfolk on Sunday night, could be a possibility. With long reliever Miguel Gonzalez not an option to join the rotation right now, according to executive vice president of baseball operations Dan Duquette, reliever Dana Eveland -- who made a spot start on Saturday -- is a candidate to start on Friday.
Showalter said he wants to wait to make a decision on Friday's starter until the three-game series with the Mariners concludes on Wednesday.
Brittany Ghiroli is a reporter for MLB.com. Read her blog, Britt's Bird Watch, and follow her on Twitter @britt_ghiroli. This story was not subject to the approval of Major League Baseball or its clubs.