SEATTLE -- James Paxton exited his 2021 regular-season debut in Tuesday's 10-4 loss to the White Sox after just four outs with left elbow discomfort, an alarming and all-too-familiar sign for the veteran left-hander, who has battled significant injuries throughout his career.

Mariners manager Scott Servais classified the injury as a forearm strain, which Paxton said was related to the strained left flexor tendon that he sustained last year, but not nearly as painful. Paxton will undergo an MRI on Wednesday.

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"It's kind of been a process through the recovery of my flexor strain from last year, throwing this offseason, going into bullpen [sessions], getting in Spring Training games,¡± Paxton said. ¡°And that process was all going well. I was going through the levels kind of building it up. And I just came into tonight, I was pretty confident that I was going to be good. But it turned out that it just was a little bit too much for what my flexor was ready for.¡±

Paxton showed visible discomfort after delivering his 24th pitch, which was well out of the zone to Andrew Vaughn at 92 mph. Paxton grunted then put both hands on his knees behind the mound. Servais and club athletic trainers emerged from the dugout and relieved the left-hander for long reliever Nick Margevicius. But it was evident in the few pitches prior to Paxton¡¯s final that something was off. His velocity dropped from 95 to 92 mph, and he was grabbing at his side.

Paxton said that he didn¡¯t begin experiencing discomfort until the second inning, and that it snowballed quickly.

¡°It just got worse and worse, and I just kind of felt it,¡± Paxton said. ¡°It got to a point where I wasn't able to throw pitches.¡±

The flexor strain last year ended his season with the Yankees after just five starts, over which he compiled a career-worst 6.64 ERA and showed significantly diminished velocity. This came after he admittedly rushed to return from back surgery that he underwent in February 2020 in order to prepare for Summer Camp with New York.

¡°There was a lot more pain than there is right now, so I'm feeling pretty optimistic that this could be a pretty quick thing,¡± Paxton said. ¡°Obviously, I don't know much right now.¡±

Nonetheless, Tuesday represented another frustrating blow for Paxton, who has been on the IL for elbow, pectoral, latissimus, finger and knee ailments since debuting in 2013.

¡°I feel like for me, it's kind of been one thing after another,¡± Paxton said. ¡°And I work really hard and do everything that I can to be out there, and I¡¯ll continue to do so. And hopefully I can catch a break and stay healthy for a good chunk of time in the future."

Paxton was initially scheduled to start Seattle¡¯s second game of the season on Friday against the Giants but was pushed back to the season¡¯s fifth game on Tuesday in order to get him more rest.?Paxton entered Tuesday on a 75-to-80-pitch threshold against Chicago, or roughly five innings.

He made just two Cactus League starts due to work visa issues, but looked stellar in both, racking up a combined 17 strikeouts over 8 1/3 innings.?

At his best, Paxton is one of the top left-handed starters in the American League, but health issues have kept him from reaching 30 starts and pitching more than 160 1/3 innings in a season (2018), a threshold he was determined to exceed in ¡®21, which seemed all the more attainable given the Mariners¡¯ use of a six-man rotation that would build in extra rest.?

Paxton worked rigorously in the offseason at the Athletic Training Institute in Bellevue, Wash., regaining his strength, polishing his mechanics, then wowing more than 20 interested clubs during a bullpen session in December that led to his reunion with the Mariners.

Paxton had multiyear offers from other clubs, but he ultimately wanted to return to the comfort of the organization that he came up in. That led Big Maple to agreeing in February to a one-year, $8.5 million deal that can reach $10 million. General manager Jerry Dipoto said after that ¡°we were the benefactors of some form of hometown discount on this.¡±

If the Mariners will be without Paxton, short- or long-term, Margevicius will take his spot in the rotation.

The left-hander is stretched out from Spring Training after competing with Justin Dunn for the fifth and final rotation spot. That would open a spot in the bullpen for candidates such as Joey Gerber and Erik Swanson, who had strong springs and were in consideration for the Opening Day roster.