Much has been justifiably made about the Mariners¡¯ bullpen and its need to upgrade heading into 2021, and while there are still significant holes, the club believes it has acquired its most high-leverage arm for the upcoming season.

That would be right-hander Rafael Montero, who Seattle acquired from the Rangers on Dec. 15 and who settled for $2.25 million to avoid arbitration, above the $1.7 million Cot¡¯s Baseball Contracts projected he¡¯d earn. Montero converted all eight of his save opportunities for the last-place Rangers last season while flashing a big uptick in velocity -- a byproduct, Montero said, of the successful Tommy John surgery he underwent in March 2018 with the Mets.

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Montero throws a four-seam fastball, sinker, changeup and an occasional slider, but after coming up slider-heavy and as a starter with New York, he¡¯s turned much more to his fastballs in the two seasons with Texas after undergoing surgery. The reasoning was rooted in simplicity -- he¡¯s got more gas. That¡¯s big for a Seattle bullpen that ranks 28th in fastball velocity, per Statcast, since trading closer Edwin D¨ªaz to the Mets after 2018.

Montero¡¯s average four-seam fastball velocity
2014: 93.0 mph
2015: 92.8 mph
2016: 93.4 mph
2017: 93.9 mph
2018: Injured
2019: 95.9 mph
2020: 95.8 mph

¡°I've had an uptick in velocity, but I've also felt like of late, I've been able to have better control, and not only that with my fastball, but I feel like I've been able to use it well off my changeup, which I also felt comfortable locating,¡± Montero said through an interpreter during the Mariners¡¯ Virtual Baseball Bash. "So it's really just been a combination of feeling more [comfortable], locating my fastball and pairing it with that changeup.¡±

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Opposing batters hit just .200 and slugged .480 against Montero¡¯s four-seamer in 2020, with 10 strikeouts. His sinker (two-seamer) limited hitters to a .176 average and .412 slugging percentage. While the swing-and-miss numbers don¡¯t jump off the page -- Montero generated a 24.7 percent whiff rate on his two- and four-seamer combined -- he also could benefit from more velocity variation between his fastballs and changeup, which touches 90 mph.

Going further, advanced metrics suggest that Montero has been an above-average reliever. His .986 WHIP ranks 15th since the start of 2019, and he had a 162 ERA+, a park-adjusted metric where 100 is league average. His 28.6 percent strikeout rate from '19-20 and 5.9 percent walk rate both ranked in the top 22nd percentile. The Mariners are also encouraged that he allowed just two homers to 72 batters faced.

¡°We like his stuff and what he brings to the mix. He¡¯s got a little experience as well, so that¡¯s a great fit,¡± Mariners manager Scott Servais said.

As is the case for most who undergo Tommy John, the surgery played a big part in Montero¡¯s psychological development. He was brought up as a starter with the Mets, weathered a significant setback, and now he believes he¡¯s untapped his most raw potential via his fastball-driven repertoire.

"It definitely changed me as a pitcher, but I think for the better,¡± Montero said. ¡°It was a good change for me. I definitely feel like I saw an uptick in my velocity after the surgery. I think I also proved [it to myself] mentally, because there was that question in my mind about how am I going to bounce back from this? What is this experience going to be like as a pitcher? And I feel like since I¡¯ve come back from it, I've really developed as a pitcher and strengthened my arm and my mind. I'm really thankful for that experience, and I've grown in multiple ways after the surgery.¡±

For these reasons, Montero has the clearest path to the Mariners¡¯ most high-leverage innings. Montero also possesses multi-innings ability, having been a starter until his surgery, though his longevity will likely be dictated by circumstance. His reverse splits are stark -- he had an opposing OPS of .776 to righties in 2020 and .525 to lefties -- which underscores the Mariners¡¯ need for bullpen versatility.

Seattle relievers combined for an American League-worst 5.92 ERA in 2020, and while general manager Jerry Dipoto acknowledges there is still room for external upgrades, he points out that Montero -- as well as Kendall Graveman and Keynan Middleton -- will fill a big part of that void.

Graveman, a former starter, is back on a one-year deal hoping to resurrect himself in the bullpen after overcoming a benign tumor in his spine. Middleton, the former Angels closer, is also re-establishing his value after undergoing Tommy John in 2018. Seattle has reportedly been in contact with representation for Joakim Soria as well.

¡°We feel like we are starting to build some stability into a bullpen that really hasn't been very stable over the last couple of years,¡± Dipoto said. ¡°And if we have the opportunity to add to that group between now and the start of the season, we will. I can't say that that will be a primary focus between now and Feb. 19. But it remains part of our focus.¡±

So, for now, the Mariners believe Montero is their man for the most important late-inning outs.