SEATTLE -- The banner waved in bright yellow with blue lettering, eerily identical to the one depicted on Scott Servais’ favorite television show, “Ted Lasso,” which in so many ways has been an embodiment of the 2021 Mariners.

“Believe,” it read from the second deck in left field, right around the area that Mitch Haniger dispensed a pair of three-run homers in the ensuing hours.

• Box score

Here the Mariners sit, just 1 1/2 games back of a postseason spot with five to play on the heels of their most dominant offensive showing in this season of surprise, a 13-4 victory over the A’s on Monday to begin the final homestand of the season.

The Mariners (87-70) have believed all along that they could end the longest playoff drought in American professional sports. Their fans have begun to catch on. And now the rest of the baseball landscape appears to be, too. Whether they play in next Tuesday’s American League Wild Card Game, which they’ll need help to reach, it’s clear that the Mariners have manifested unapologetic belief in themselves.

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“That’s what's fueling us and firing us,” Haniger said. “I mean, as a team, we want to win every single game the rest of the season, come in every day prepared to win. That’s the goal, to control what we can control, to go out there, take it one game at a time, one pitch at a time. And at the end of the day, I think if we do what we need to do, hopefully we're in that Wild Card Game.”

The Mariners still need help in the form of losses by the teams in front of them -- New York (89-67), Boston (88-68) and Toronto (87-69), all of whom were idle -- and they’re running out of time. But they continue to do their part by taking care of business after an 8-2 road trip. They are now 16-8 in September and have won a whopping 10 games in a row against the defending AL West-champion A’s, who fell to 3 1/2 games back of the final postseason spot.

Seattle also continued its dominance over Oakland starting pitcher Cole Irvin, who dropped to 0-5 with an 8.69 ERA in five starts against the Mariners this season and had candid comments about the slight that he made after his first outing against them on May 25. Irvin at the time said, “A team like that should not be putting up 10 hits against me or anyone.” And after giving up five runs and lasting just three innings on Monday, he recognized that it might’ve created bulletin-board material.

“Baseball has a way of humbling itself,” Irvin said. “To be honest, it goes back to the comments I made early in the season. They’re hungry every time. They want to face me and beat me into the ground, and they did that every time I faced them. My job is to be able to execute pitches, and I felt like I have at times, but not enough when they want it more. I wasn’t good enough. It’s embarrassing on my end.”

The Mariners are now two wins shy of tying the 89 they compiled in 2018, their final season before embarking on this multiyear transition to a younger roster. That win tally also remains the highest in the six seasons under GM Jerry Dipoto and manager Scott Servais, and it appears that they can very well pass it over the final five games, especially if they keep playing the way that they have. They will reach 90 wins, a mark that would’ve been unthinkable in Spring Training, by going 3-2 the rest of the way.

Monday’s winning formula was yet another reminder that there is no quit in this team, especially after Chris Flexen surrendered three runs in the first inning. That quickly felt like eons ago after Seattle tied it in the third on an RBI double by Ty France and a two-run, two-out single by Luis Torrens.

But it was the fourth inning in which they blew things open, thanks to Haniger, who now has six career multihomer games, including three against Oakland. With six RBIs, the 30-year-old veteran also set a new career high, with 94.

He blew past his previous career-high homer total of 26 over a month ago. And he reclaimed the team lead from Kyle Seager, who has 35. Only eight big leaguers, including MVP contenders Shohei Ohtani, Vladimir Guerrero Jr. and Fernando Tatis Jr., have crushed more homers this season than Haniger.

Those power numbers underscore just how valuable he’s been to a Mariners club that, for the most part, plays in tight games night in and night out. For all of Seattle’s surprising success this season -- a huge step forward by J.P. Crawford, the consistency of Ty France, Seager’s possible swan song here and more -- Haniger merits as much credit as any player for helping put them in this position.

They will continue to need his efforts in this final run, and even Servais has admitted that the team is running on fumes physically. But there is a whole lot of “believe” left for this final week.