MILWAUKEE — The Dodgers will be without their top pitching prospect for an extended period of time after announcing on Tuesday that right-hander River Ryan will undergo Tommy John surgery. Full Tommy John surgery typically requires a recovery timetable of between 12 and 18 months.

The 25-year-old Ryan, ranked by MLB Pipeline as the club’s No. 3 prospect and No. 99 overall, excelled in four starts after his first big league callup on July 22. The former 11th-round pick posted a 1.33 ERA and struck out 18 batters over 20 1/3 innings but was forced to leave Saturday’s start against the Pirates early with tightness in his forearm.

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.

Ryan underwent an MRI on Sunday, after which the Dodgers determined he would miss the rest of this season. Further evaluation by team doctors in recent days provided more clarity.

“Even for me, talking to River, he’s been through trials and knows how to handle it mentally,” manager Dave Roberts added before Tuesday’s game against the Brewers. “He’s going to miss being with the guys, but this is the surgery that they’ve got a pretty good handle on, and so we expect a full recovery. But it’s disappointing.”

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.

Walker Buehler (right hip inflammation) is expected to be activated off the 15-day IL and start on Wednesday against the Brewers. Yoshinobu Yamamoto (60-day IL, right triceps tightness) is set to join the Dodgers in St. Louis on Friday and throw two innings in a simulated game. He threw a 40-pitch bullpen session on Tuesday.

Roberts also pointed to Landon Knack and Justin Wrobleski as options on Los Angeles’ depth chart. Knack has a 3.07 ERA in nine games (eight starts) in the Majors this season. Wrobleski, who’s with Triple-A, has a 4.05 ERA in 20 innings over four starts for the Dodgers this season.

“It’s been challenging,” Roberts said over the weekend. “Personally, I feel for each guy, because it’s their careers and I know how hard it is. Most recently with River and how disappointed he is, but it’s one of those things that there’s some sensitivity in the sense of feeling it, but also making the guys available feel like they’re the best options. I think they’ve done a really good job of picking up the pieces, and the guys who are available are giving us the best chance to win.”