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.

• Offseason checklist: Mariners' needs, moves

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 Díaz 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.”

• Mariners’ Virtual Baseball Bash continues this week

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.