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JUPITER, Fla. — What set up to be a defining spring for former first-round pick Zack Thompson's place with the Cardinals as they turn toward youth and look to the next wave of pitching prospects has encountered a delay due to injury.

Thompson will not be able to throw for three to four weeks due to a strain of his lat muscle along the left side of his back, Cardinals president of baseball operations John Mozeliak said Sunday. The prescribed inactivity for most of March assures the lefty will miss more than a month before returning to games as the Grade 1 tear calms and heals. He will then need to regain arm strength before advancing to the mound and into competition.

"We'll reevaluate from there," Mozeliak said. "It's obviously disappointing in terms of you want to come into camp, be ready to go. For him, this is definitely a setback. ... Lat strains can be concerning because sometimes it can just linger and be prolonged."

Thompson, 27, felt discomfort around his left shoulder and side within the past week and tried to address it and work through it.

He had an MRI taken Saturday that revealed the cause of the soreness.

Thompson appeared in one game and pitched one inning this spring for the Cardinals. He's coming off a disappointing season that was spent mostly in Class AAA Memphis. In 2023, Thompson found success in relief with the Cardinals as he struck out 72 in 66â…“ innings in the majors, but for an encore, he had difficulty maintaining consistent velocity or success.

He was limited to five games (two starts) in the majors in 2024, and at Triple-A, he went 4-6 with a 4.40 ERA in 21 games (20 starts). He did strike out 108 in 90 innings against only 68 hits allowed, but he also walked 56 batters.

The Cardinals had Thompson on a starter's schedule to increase pitching count as this month progressed, though he was competing for a role as either a starter, if needed, or a reliever in the big league bullpen. The Cardinals also have lefty Matthew Liberatore, right-hander Kyle Leahy and right-hander Gordon Graceffo in a similar place to start the month — stretching out as starters with multiple innings in games but also auditioning for any of the openings in the bullpen.

The 19th overall selection in 2019, Thompson moved to Florida this past offseason to prepare for this spring training in the warmer weather and closer to trainers.

"There are a lot of young guys coming," Thompson said in early February as he worked out at the Cardinals facility before camp opened. "I’m at the weird stage. So I've just got to be ready for spring to start here next week and go attack guys and see what happens."

The timing of the injury erases Thompson's ability to appear in Grapefruit League games before the Cardinals leave Florida for the regular season.

He will be reevaluated about the time the Cardinals head north. If cleared to resume throwing, he would be that many weeks behind the other pitchers and require a spring training-like buildup before competition.

"So far for the Cardinals camp we've been healthy," Mozeliak said. "Fingers crossed he can get back out there."