"I think he knows now that he's starting today," Showalter said after McFarland pitched the Orioles to an 8-3 win against the Rangers on Tuesday.
The lefty needed just 68 pitches to give Baltimore five innings of two-run ball and pick up his first win of the season in the second start of his career -- his first since June 28, 2013 against the Yankees.
McFarland struck out two batters and didn't walk any, letting his defense do much of the dirty work behind him.
"I threw a lot of strikes and basically just let the defense help me out," he said. "I got a lot of ground balls, a lot of double plays and I can't stress it enough that the defense really helped me out."
Showalter said McFarland is an option to start again on Sunday and beyond, but after the win he seemed to soften his stance on that option.
McFarland has been devastatingly effective out of the bullpen -- his 0.57 ERA in June was the best in the Majors among pitchers that threw at least 15 innings -- and Showalter views him as a versatile option in relief. Pitcher Kevin Gausman is the most likely option to start on Sunday.
"He did exactly what we hoped he'd do," the manager said. "He makes our bullpen better, but it's nice to know that you have that type of option there."
McFarland's day ended when he got Rangers shortstop Elvis Andrus to ground into a double play to end the fifth inning. McFarland hadn't yet hit 70 pitches, but Showalter said that was as far as the O's wanted him to go.
He helped the Orioles get out to a commanding 8-2 lead and had Alex Rios and Adrian Beltre coming up to start the sixth. In a new role, McFarland shook off some early nerves and helped Baltimore win its second straight against Texas.
"I'm always going to be a little nervous before," McFarland said. "Even when the phone rings in the bullpen I get a little stomach dropping, but it's all good emotions that I should be feeling and I was just excited for today."
Brittany Ghiroli is a reporter for MLB.com. Read her blog, Britt's Bird Watch, and follow her on Twitter @britt_ghiroli. David Wilson is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.