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Seattle Mariners Betting Preview 2021

The Seattle Mariners continue to hold one of baseball’s most dubious distinctions. They’re the only team in MLB history never to reach the World Series. In fact, they haven’t qualified for the playoffs since 2001 when they reached the ALCS, losing to the New York Yankees in five games after winning an American League record 116 regular-season games.

Mariners’ fans know all too well the team’s lack of success, but there is a renewed sense of optimism in the Emerald City, one that’s vastly different from 2013 when they broke the bank to acquire Robinson Cano.

2020 Results

In the pandemic-shortened season that was 2020, the Mariners were never able to climb above .500. Not once. Through eight games, they were 4-4, but 2 1/2 weeks later, found themselves at 7-18, enduring losing streaks of three, four and seven games.

Seattle was able to close out its campaign on a good note, though, going 14-11 in their last 25 to finish the year 27-33 and in third place in the AL West. Oakland won the division going away, seven games ahead of second-place Houston, who also secured a playoff spot, and nine in front of the Mariners.

Seattle bested the Angels and Rangers, who finished 10 and 14 games back of the A’s, respectively. The Mariners haven’t finished higher than third in the American League West since 2016 when they closed out nine games back of Texas, which had an AL-high 95 wins.

Key Additions 

Arguably the biggest addition to this Mariners team is not new, but familiar. James Paxton returns to Seattle after a two-year stint with the Yankees, agreeing to terms on a 1-year/$8.5M deal in mid-February. “The Big Maple” began his MLB career with the Mariners after the club selected him in the fourth round of the 2010 MLB draft, and returns to help solidify the top half of the rotation.

Seattle took a risk on another prominent free-agent signing when it acquired Ken Giles in free agency on a 2-year/$7m deal. Giles underwent Tommy John surgery in September of last year and will miss the entire 2021 season with the hope that he is healthy enough to fill the closer role in 2022.

Giles has pitched for the Phillies, Astros and Blue Jays, once converting 34 consecutive save opportunities between 2017-19 and setting a Toronto record by recording 26 consecutive relief appearances with at least one strikeout.

Giles’ absence makes the acquisition of Rafael Montero key by default. Seattle traded for the reliever this offseason, sending Jose Corniell and a player to be named later to Texas, in exchange for the righty that the Mariners hope can fill the closer role in the interim. In 2019, Montero went 2–0 with a 2.48 ERA and 34 strikeouts over 29 innings and then went perfect in eight save opportunities last year.


Not having an established closer like Giles will likely have an impact early on, but the team signed him knowing the risk/reward factor much like the Brooklyn Nets with Kevin Durant last year. But Seattle’s young core is intact and there are high hopes in the Northwest for Montero.

All in all, nary a noteworthy big loss, unless you count missing out on players like Brad Miller, Tommy LaStella and Taijuan Walker in free agency.


The Mariners were one of the first big-league clubs to institute a 6-man starting rotation and there’s a chance they could be returning to that mindset in 2021. It’s entirely possible that Paxton could be joined by Marco Gonzales, Yusei Kikuchi, Justus Sheffield, Justin Dunn and Chris Flexen.

Paxton is the big name, but Seattle’s ace is Gonzalez and the lefty will get the ball Opening Day against the Giants. Gonzales led all of baseball with a 9:1 strikeout-to-walk ratio last year, going 7-2 over 69.2 innings with a 3.10 ERA.

Kikuchi improved his strikeout ratio from 6.5 in 2019 to 9 last year. The 29-year old Japanese lefty worked on his cutter and delivery last year, just his second season in the big leagues.

Sheffield was the big prize in the deal that originally sent Paxton to New York in 2018, and in 55 innings pitched last year, gave up just two home runs. The 24-year old matched Gonzales with a team-high six quality starts in 2020.

Whether they do deploy a six-man rotation could depend on Flexen. The big righthander spent 2020 in Korea, where he was one of the top pitchers in the KBO. Flexen is a former Mets prospect who started 11 games over three years in Queens.


While there were positives to come out of 2020 with hope to build on, the Mariners did finish in the bottom third in major offensive categories like hits, runs, home runs, batting average, slugging and OPS. They finished third in MLB in steals, however, though only one player (Dylan Moore) registered double-digits in that category.

The Yankees’ Luke Voit led all of baseball with 22 HR, whereas Kyle Lewis’ team high of 11 was good for a tie for 40th. More on Lewis in a moment, but this team will have to pick it up offensively considering the uncertainty in the back half of the rotation and much of the bullpen. Kyle Seager and his elite eye at the plate should be counted on in the box, striking out just 31 times in 201 at-bats last year.

3 Key Players 

Lewis is by far the team’s offensive standout. The reigning American League Rookie of the Year led the team in homers, hits and walks last year and is one of the most talented young players in the game. If you’re looking at a sportsbook for a longshot to win the AL MVP, Lewis’ +6000 odds are quite intriguing.

JP Crawford has matured into an ideal leadoff hitter and his defense earned him a Gold Glove last year, the first for the 25-year-old former top draft pick. Crawford wasn’t the only recipient, however, as Evan White brought home a Gold Glove as well at first base. The 24-year-old led all AL first basemen with seven Defensive Runs Saved — two more than any other AL player at the position.

Mitch Haniger might just hold all the keys to this offense. After the departures of Robinson Cano, Nelson Cruz, Edwin Diaz and even Paxton, Haniger was thrust into a leadership role on a young,  green team. However, the 30-year old missed half of 2019 with a ruptured testicle and sat out all of last year dealing with the pandemic and various other surgeries.

Haniger is expected to slot into the 5-hole and provide more pop to a lineup otherwise reliant on speed and contact.

2021 Seattle Mariners Schedule Breakdown

Seattle kicks off the 2021 campaign with six straight at home against San Francisco and the White Sox. The Mariners won’t face another AL West team until April 16 when they welcome the Astros to town. They were 20-20 against the rest of their division last year and that could improve if Houston falls back as many expect.

According to the sportsbook, the Mariners aren’t expected to fare much better than a season ago, garnering odds of +7500 to win the World Series. As far as the division goes, Oakland, Houston and Los Angeles all have shorter odds than the Mariners who come in at +3000 to be crowned AL West Champs.

In terms of regular-season wins, has the over/under at 72.5 games for the Seattle Mariners. If you are new to sports betting, please check out our MLB betting hub page to get started.

This is not a team that jumps out at you in terms of star power. The Mariners will rely on defense, consistency and an added year of experience for their young players. They didn’t overspend in free agency and probably won’t until their core is actually ready to compete. They do have the potential to turn some heads and surprise AL followers, but they’re still likely another year or two away.

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