LOS ANGELES — Dodgers right-hander exited his fourth major league start with two outs in the fifth inning with right forearm tightness, an ominous sign for the rookie pitcher.

Ryan had a 3-0 lead and nobody on base, just one out shy of qualifying for a potential win. But with a 3-1 count on Michael A. Taylor in the fifth inning, Ryan exited with trainer Thomas Albert, and appeared to be shaking his hand after a pitch on the SportsNet LA broadcast.

After the game, Ryan said he first felt tightness in the third inning, and used a massager in the dugout to loosen up. The feeling returned in the fifth.

“The last two pitches, it really started to tighten up on me a little bit,” Ryan said. “I begged Doc to let me keep going, but they pulled the plug.”

Against the Pirates on Saturday, Ryan allowed only three singles and a double in his 4⅔ innings, walked none, and struck out four. He was incredibly efficient to that point in the game, having only thrown 56 pitches.

An MRI exam is scheduled for Sunday, and Ryan will be placed on the injured list.

“We’ll know more tomorrow, but obviously when you see a guy go for his forearm, it’s always disconcerting,” manager Dave Roberts said. “Just with the calendar — I don’t want to speculate too much, but we’re running out of time.”

Ryan missed the first two months of the season with shoulder soreness, and essentially used June to stretch out in the minors like it was spring training, topping out at five innings in Triple-A.

Called up for his major league debut on July 22, Ryan has impressed in his four starts with the Dodgers, allowing only four runs (three earned) in 20⅓ innings for a 1.33 ERA, with 18 strikeouts and nine walks.

“Everyone likes his stuff. The fastball plays, the breaking ball. He has different weapons to get guys out. I think he’s just handled himself really well. Very confident,” Roberts said before Saturday’s start. “Just trying to manage his workload. You look at the most he’s pitched professionally is in the big leagues. That just doesn’t happen very often. It’s rare. But he says he’s good physically.”

Roberts said Landon Knack will be called up from Triple-A on Sunday, to pitch out of the bullpen for now. Knack started for Oklahoma City on Tuesday, striking out eight in 5⅔ innings.