McFarland was chosen over Kevin Gausman, who will be available in relief.
McFarland will be making his first major league start. He hasn't worked more than 4 1/3 innings in any relief appearance.
McFarland is 1-0 with a 4.14 ERA in 18 games.
Also, Showalter said second baseman Brian Roberts most likely will remain on the disabled list until Sunday.
More to come.
Update: Manager Buck Showalter was asked for plate umpire Will Little's explanation for ejecting Manny Machado in the fifth inning.
"Just told me what he said," Showalter repled.
"I think Manny was just a little frustrated and thought he might have fouled it, from what I understand. I haven't spoken to him yet. I asked him why and he gave me a pretty good explanation."
Machado's streak of playing every inning ended tonight at 1,206.
"Start another one," Showalter said. "I've seen 10-15 year veterans get frustrated with some calls. It's all lessons learned. He's 20 years old. I don't find anything surprising about him playing a lot. When I take people out of the game way ahead or way behind, he's a guy who's going to have to wait his turn a little bit."
Crew chief Ted Barrett, speaking on behalf of Little, explained the ejection to a pool reporter.
"It was an expletive, and then something directed at (Little), so that's an automatic ejection," Barrett said. "He knows that."
The fifth inning was filled with bizarre moments, including Adam Jones being called out at second base for surrendering the bag. He had been ruled safe initially.
"It seemed like a long time ago, doesn't it?" Showalter said. "What were some of the bizarre things in your mind, besides Adam being called safe? You can see how it happens. He calls safe and nobody can hear because the crowd is reacting to the close play at first. I've seen guys not know the guy was considered off the base. I've seen umpires miss calls. I've seen players and managers make mistakes. Orioles won and get some sleep and do it again tomorrow."
Barrett talked about the Jones play at second.
"He (umpire Mike DiMuro) had ruled that (Mike Aviles) didn't touch the bag," Barrett said. "Jones apparently thought he was out, so he was actually out for abandonment when he left the base. He apparently thought he was called out."
Because of the crowd noise?
"Possibly," Barrett replied.
Miguel Gonzalez allowed three runs and nine hits in 6 2/3 innings to improve his record to 6-3.
"He made some adjustments as he went along," Showalter said. "He was elevating the ball alot for Miguel, but it's a tribute to him. A lot like Tilly (Chris Tillman). He finds a way. I think it was 50-something pitches in two innings and you didn't like the chances of him pitching seven innings, but he did.
"That challenges your concentration level there with the rain delay and what have you. But Brian (Matusz) got a big out and I thought Troy (Patton) threw the ball well again. And Darren (O'Day), not having to get anybody up there through all of that... We stayed away from a lot of people we wanted to stay away from tonight."
Showalter announced that McFarland would start Friday night. And here's why:
"It's his turn," Showalter said. "He's four days rest. Be ready to go 80-plus pitches tomorrow, at least, and hopefully pitch five or six innings. The Yankees will have something to say about that. We're in a position to cover whatever innings. We wanted to see how we did tonight.
"Mac is throwing the ball well and deserves an opportunity. We had a couple options, he was one of them and that's where we're going to go tomorrow."
So much for the idea that second baseman Brian Roberts will come off the disabled list Friday.
"Think Brian got through both games (at Triple-A Norfolk) tonight, did well," Showalter said. "I think the plan, like I said, is sometime this weekend. Either Saturday or Sunday. Probably Sunday more likely. We'll see."