The Cubs placed Keegan Thompson on the 15-day injured list Saturday, the day after the 27-year-old surrendered 5 runs in 3.2 innings to the Brewers.

The injury is listed as low back tightness, something Thompson battled throughout his Friday outing. But overall, the second-year right-hander is dealing with fatigue from his largest workload in years.

Thompson now has 104.1 innings under his belt. The only time he has pitched more in a season was 2018 when he tallied 129.2 innings across two levels in the Cubs farm system.

“The main thing is like you’ve seen the last couple outings, I think the fatigue in the lower half, tightening up the back — all that stuff,” David Ross said. “It’s not shocking that it happened. It’s affecting his pitching. Is it something we’re super long-term concerned about? No, it’s not.”

With the playoffs all but out of the question this season, the Cubs are taking the prudent approach with a young starter who figures to be a major piece of the team’s pitching staff in 2023 and beyond.

The Cubs are confident some time off will be good for Thompson to get his legs and lower half back to full strength and ensure the lingering back issue doesn’t turn into anything more serious.

“As much as he wants to keep going, I think it just makes some sense,” Ross said.

For his part, Thompson admitted he is feeling some fatigue but he refuses to use that as an excuse.

This is Thompson’s first full season in the big leagues and he has spent much of the year in the rotation. He has found success (3.97 ERA) while also learning a lot about what it takes to be in the rotation for a 162-game season.

“It’s just so emotionally and physically taxing playing six months with two months of spring and being mentally locked in that long,” veteran starter Drew Smyly said. “I don’t think fans and other people outside of the game realize how emotionally taxing it is as well as physically just to continue to try to pitch at a high level for that long.

“Your rookie year, I think you’re trying to just tread water and stay afloat and just get by.”

The Cubs have been cognizant of managing Thompson’s and Justin Steele’s workload all season so this breather can be a sliver lining for Thompson.

As for who replaces Thompson in the rotation, the Cubs haven’t determined that yet but the team called up Anderson Espinoza to add another fresh arm on the pitching staff.

“We’ll probably grab somebody from the minor leagues,” Ross said.

The Cubs don’t have an off-day until Sept. 1 and they have a doubleheader against the Cardinals Tuesday at Wrigley Field.