SEATTLE -- If last night felt like it’s all becoming very real, consider that the next time Luis Castillo takes the mound after Saturday’s dominant start that led to a 5-1 win over Oakland, it’ll be in Game 1 of the AL Wild Card Series next Friday. 

No, the Mariners haven’t formally announced their postseason rotation order, and the opponent and venue are still to be determined before the regular season ends Wednesday. 

There’s also a real possibility that it could be right here at T-Mobile Park, though the Mariners will need some help for that to happen. Yet in contrast to the uncertainty of who, when and where, one certainty as the Mariners eye their first postseason game since 2001 is that they’ll likely be doing so behind their prized workhorse.

Standings update: 87-70 (second AL Wild Card spot)

Games remaining: 5

AL Wild Card: The Blue Jays lead the Mariners by 1 1/2 games for first berth; the Mariners lead the Rays by 1 1/2 games for second berth

Seeding battle: Seattle owns the tiebreaker over Toronto (5-2 in the season series); Tampa Bay owns the tiebreaker over Toronto (10-9) and Seattle (5-2)

If all three teams tie: Tampa Bay earns the top spot by owning tiebreakers over both Toronto and Seattle, and Seattle earns the second spot by owning the tiebreaker over Toronto

Making his first home start since signing a five-year, $108 million extension last weekend, Castillo again showed why he’s as key of a piece to this postseason push as any player on the roster. He carved through an A’s club that, despite its last-place standing, had his number in two starts in Oakland since he arrived in a Trade Deadline blockbuster, striking out eight and retiring each of his final 16 batters.

“I really truly in my heart felt like I [helped] ... get to this goal that they were trying to,” Castillo said through interpreter Freddy Llanos. “But like I say, I’m always positive. It doesn't matter if it's a regular season game or if it's postseason or even a series finale, I'm always going to go up there with a positive mentality and just give it my all.”

Castillo’s lone blemish came in the first inning, when he surrendered a leadoff single to Tony Kemp, walked Sean Murphy then gave up another single to Jordan Diaz that, aided by an error from left fielder Jesse Winker, led to the A’s only run. And Seattle’s bats immediately responded with a three-spot in the first that gave Castillo the cushion he needed to go scoreless over his final five innings. 

It’s too early for the Mariners to announce Castillo as the Game 1 starter, but manager Scott Servais said this week that they will stay on turn through the regular season finale, which would set up for Castillo, Robbie Ray (who pitches Sunday) and George Kirby (Monday) to start the best-of-three Wild Card Series next weekend. But there is wiggle room if the Mariners wanted to skip Gilbert’s turn on Wednesday and start him in Game 3 instead.

Having one of the best starting staffs in the tournament is a luxury for Servais, especially in a shorter series -- and especially with an arm like Castillo’s to set the tone. 

“It's the consistency of your starting pitching,” Servais said. “And you look through the years, the teams that have had sustained success, it’s driven by starting pitching. Way back in my day, it was the Atlanta Braves when they won 14 [division] titles in a row.”

Castillo averaged 97.1 mph on his four-seam fastball on Saturday while topping out at 98.8 mph, riding the heater to half of his strikeouts. The other four punchouts came via his slider, which he landed for strikes when needed and generated out-of-zone chases.

But those weapons were predicated on the two-seamer (also classified as a sinker), the fastball with arm-side run and which he used early in counts.

“The game kind of talks to me and ending on how the batter comes is where I decide what kind of pitch it is,” Castillo said. “The slider and the two-seam fastball that I kind of use to kind of have that movement.”

Castillo’s velocity and movement on his fastballs and spin on his breaking pitches make him as prototypical of a postseason pitcher as there is. And now it’s finally on the horizon. 

“His slider is really good. It's a pitch he should probably lean a little bit more,” Servais said. “We saw it today with the movement on his fastball. It's a heck of a combination.”

In 11 starts with Seattle, Castillo has a 3.17 ERA and held hitters to a .226/.286/.350 (.636 OPS) slash line, numbers that were mostly exacerbated by his most recent two starts against the Royals, during last Sunday’s epic collapse, and these A’s, starts over which he gave up a combined nine earned runs in 10 innings. 

But he showed in his final regular-season start that, if there were any kinks, he’s firmly back on track.