LOS ANGELES — Dodgers right-hander River Ryan has impressed over his first four Major League starts. But as he worked on another gem on Saturday at Dodger Stadium, Ryan’s night ended much sooner than expected, as he left the Dodgers’ 4-1 win over the Pirates in the fifth inning alongside a trainer.

Ryan, who’d allowed just four hits over 4 2/3 scoreless innings, was immediately seen shaking his right arm in discomfort, and the Dodgers later announced that he’d left with right forearm tightness.

The diagnosis is a blow to the Dodgers and the young pitcher, who was outdueling rookie phenom and National League All-Star Game starter Paul Skenes. Dodgers manager Dave Roberts confirmed after the game that Ryan will get further testing on Sunday, but will land on the injured list.

“Hopefully we have some good news, and we’ll see,” Roberts said. “Obviously, we’ve got to be careful with him. We’ll know more tomorrow. But obviously, when you see a guy go on his forearm, it’s always disconcerting.”

Ryan said he started to feel discomfort in his right forearm in the third inning, but didn’t think too much of it. In between innings, Ryan worked on the forearm with a massage gun to keep it loose. But in the fifth, as he fired off a couple pitches to Michael A. Taylor, the coaching staff noticed Ryan wincing in pain.

The rookie lobbied to stay in the game, but the Dodgers weren’t going to take any chances.

“I definitely didn’t want to come out of the game,” Ryan said. “I wanted to finish. But they saw me grimace a little bit on the mound, and they’re not going to take any chances. So they decided to call it then and there.”

Losing Ryan is the latest hit to a Dodgers team that has been decimated by injuries this season. In fact, a big reason Ryan made his Major League debut much earlier than expected — and just two years after becoming a full-time pitcher — is because the Dodgers have had 11 starters land on the injured list at least one time this season.

“This is probably the highest number I’ve dealt with. And really quality arms,” Roberts said. “You try to do everything you can to prepare them, take care of them, give them extra rest. Right now, it’s just happening a lot, certainly to us. Fortunately, we’ve got a lot of depth and a lot of good ballplayers. We’ll just try to backfill. But I just feel bad for River. Because he was pitching such a good ballgame, and was just excited, kind of feeling his energy.”

In his first four starts, Ryan excelled, posting a 1.33 ERA. Ryan is also a big part of the Dodgers’ future in the rotation, as he is ranked the No. 4 prospect in their system, per MLB Pipeline.

With Ryan set to miss some time — and perhaps as much as the rest of the season, given how many games remain on the schedule — the Dodgers will turn to Walker Buehler to fill in that spot in the rotation. Buehler made three rehab starts with Triple-A Oklahoma City and is scheduled to pitch on Wednesday against the Brewers in Milwaukee.

If the Dodgers are in need of a sixth starter during tough stretches, they can also turn to Landon Knack, who will be activated on Sunday and serve out of the bullpen, or Bobby Miller, who continues to work through his mechanics in Oklahoma City.

Though there are options, it was a disappointing way for the Dodgers to end a day in which they scored four runs of Skenes, the most he’s allowed in a start in his big league career, and secured a series win against the Pirates.

“Obviously this was a big setback, but I think for River himself, [he’s] just really opened eyes, and [we] know that he can pitch here and really do some special things,” Roberts said. “I think that’s a takeaway right now. … Having some other guys back that have been there, we have some reinforcements. They’ve got to be good.”