Johnson will be making his fifth major league start. He was 4-0 with a 2.11 ERA in 12 games last year, including four starts, with 18 walks and 46 strikeouts in 38 1/3 innings.
Johnson has made three starts for Triple-A Norfolk this season, including two on his injury-rehab assignment. He's allowed eight runs and 17 hits in 16 1/3 innings, with four walks and 19 strikeouts.
Johnson began the season on the 15-day disabled list with a strained right lat.
Showalter didn't place the blame for tonight's 6-2 loss to the Royals on Freddy Garcia's bad fourth inning, when he served up a pair of two-run homers.
"We scored two runs tonight," Showalter said. "Freddy gave us a chance to win. It was good to see. I had some guys I wanted to get out there tonight. We had a little trouble keeping the ball in the park.
"I thought Mac (T.J. McFarland) threw the ball good. He had a ball that hit the foul pole and another ground ball that we probably should have caught that snuck through. (Garcia) paid for the few mistakes he made with the ball leaving the park. I think it's more of a two-run thing."
Garcia came out after only 77 pitches.
"There's a lot of things," Showalter said. "Mac needed to pitch and I thought it was a good spot in the order. He pitched well. Wanted to get (Alex) Burnett out there. And Freddy gave us a good chance to win. He could have continued easily - pitch count, pitching on his normal day. He continues to give us a chance to win in those two outings. It's kind of the needs of everybody, including Freddy."
Former Oriole Jeremy Guthrie held the Orioles to two runs (one earned) over six innings while improving to 5-0.
Showalter wasn't interested in comparing Guthrie tonight to the pitcher he managed in Baltimore.
"I didn't pay much attention to that part of it," Showalter said. "We've kind of turned the page. Jeremy did some good things for us. I'm glad he's doing good things with his career and life. Jeremy had some good outings for us, had some times he was challenged. Just like every pitcher does. That wasn't a point of emphasis for me. But he pitched well enough for them to win tonight. He gave them six innings. We thought he might continue, but I guess not."
McFarland was pitching for the first time since May 1 and the second time since April 25.
"He pitches a lot, just not in games," Showalter said. "He does a lot of work in sim situations, just facing hitters. I'm impressed the way he pitches with not consistently getting the ball in his hand. It's kind of the life of a guy that's there to cover your starter.
"That was another thing. I really wanted to get T.J. out there in some form or fashion if the game allowed it. I had two or three relievers I was not going to pitch tonight. You win two games using those guys, we're not going to pitch the same guys every night weeks at a time. Just can't do it."
The Orioles were frustrated all night with plate umpire CB Bucknor. Ryan Flaherty threw his bat and helmet after being called out on strikes to end the sixth.
"I understand some frustration," Showalter said. "It was obviously a ball there. You guys watched the same thing I did. Not very entertaining, watching the inconsistency of that. It's not something that's real new. He was on base two times, probably should have been three or four if you get some cooperation with some other factors.
"Ryan's been playing the heck out of second base. I saw all the work he did today. He's giving himself a chance. He's having a little better at-bats, just missing some balls. I know it's frustrating for him."