DENVER -- After struggling through lower back pain for the last 3 1/2 weeks but not finding answers, Rockies left-hander Austin Gomber learned this week he has pars defect -- a stress reaction in the lower back. He will miss the remainder of the season.

Gomber, 27, the key acquisition in the trade that sent star third baseman Nolan Arenado to the Cardinals, went 9-9 with a 4.53 ERA in 23 starts covering 115 1/3 innings in his first true opportunity as a Major League starter. He had fulfilled starting and bullpen roles with the Cardinals in 2018 and 2020 (he didn’t pitch in the Majors in ’19 because of right biceps tendinitis).

Gomber will not require surgery, and after a rest period will begin preparing for 2022 -- when he will be considered a key member of a rotation that has shown promise.

“Proving to myself that I can be a successful starter in the Major Leagues was a positive,” Gomber said. “I have a list of things that I want to work on and put into play for next year, for sure.”

After a rough debut at home against the Dodgers (seven walks in three innings on April 4), Gomber put together a solid season at Coors Field -- 5-1, 2.09 ERA in nine starts. He missed a month with left forearm tightness. While he was mostly solid after his July 21 return, there were hiccups. He gave up three home runs at Wrigley Field on Aug. 25, and walked six in 4 1/3 innings in his last start, Tuesday at Texas.

“He felt good enough to pitch in Chicago and then from there, it got progressively worse,” manager Bud Black said. “He threw his bullpen and pitched in Texas, and felt a little discomfort. We still thought it was manageable. He saw some doctors and then ultimately when we got back here we got a scan done, and they found this.”

Gomber said his understanding is the injury was an old fracture, and the stress led to pain. The injury is on the right side of the back. That means he felt it each time his right foot landed as he threw a pitch.

The injury may not be an answer, but it could contribute to a hypothesis that teams MLB-wide are developing: Going from 60 games last year because of the COVID-19 pandemic to a full 162-game schedule has been rough on pitchers.

Jon Gray, who threw a 45-pitch simulated game on Saturday, is working back from a right forearm strain for the second time. Kyle Freeland, who missed the beginning of the year with a left shoulder injury, had a left hip impingement flareup during his last start. Black is looking for an opportunity to rest Germán Márquez, who was set for his 28th start on Saturday but had shown signs of wear.

“I don’t know exactly what caused it, necessarily,” Gomber said. “I think a bunch of factors that could play into it. I must have done something specifically in one of my starts in the past month, or swinging or doing something that causes a reaction.

“Obviously my workload, from the previous years has been increased significantly, so I'm sure that probably plays a role into it. My first year playing here plays a role, and there are probably a lot of things that play into it.”

The injury announcement was one of several on Saturday:

• Gomber will be placed on the 60-day injured list, which will make room for righty Ryan Feltner, the Rockies’ No. 22 prospect, per MLB Pipeline, to make his Major League debut on Sunday afternoon against the Braves. Feltner, 25, is a combined 8-3 with a 2.62 ERA between High-A Spokane and Double-A Hartford.

A pitcher who relies on his fastball-slider-changeup mix, with the fastball being a Major League average pitch, Feltner was a fourth-round MLB Draft pick out of Ohio State in 2018. The Rockies' coaches, with their observations and readings from the technology in their pitching lab in Scottsdale, Ariz., suggested a shorter arm sweep and more efficient front-side movement during 2019 fall instructional ball.

“To be able to work on some of these things at home during quarantine, during time I wouldn't have, that was a blessing in disguise for me, personally,” Feltner said. “My brother used to play baseball and we would throw together, and we had a little garage gym set up.”

• The Rockies placed rookie Connor Joe -- who had grabbed regular playing time in left field -- on the 10-day injured list with a mild right hamstring strain that he sustained during Friday night’s 4-3 victory over the Braves. Right-handed-hitting Joshua Fuentes was called up from Triple-A Albuquerque.

• The Rockies activated righty Jordan Sheffield from the 60-day injured list. Sheffield had been out with a right lat strain.