SEATTLE — There are no “must” games until losing eliminates you, but Tuesday certainly rated a near-must for the Mariners in the uphill pursuit of their first postseason berth in 15 years.

So no, this…this fiasco of a 10-2 loss to the Toronto Blue Jays in front of their traveling legions at Safeco Field…doesn’t eliminate the Mariners. Not mathematically, anyway.

But it’s hard to fathom a more demoralizing evening.

The Mariners unraveled in the fourth inning as Toronto scored eight runs through a combination of defensive mistakes, long-ball pop and just about everything else.

And it came out of nowhere.

The Mariners had a 2-0 lead, thanks to a pair of gifts from Toronto second baseman Devon Travis in the third inning. The second one turned Robinson Cano’s two-out grounder into a two-run triple.

Hisashi Iwakuma (16-12) seemed to be rolling, too. Three shutout innings, and he started the fourth by striking out Edwin Encarnacion and inducing a foul pop from Jose Bautista.

Everything turned at that point.

Catcher Chris Iannetta over-ran the ball and failed to catch the pop. An error. Given a second chance, Bautista extended the at-bat for seven more pitches before serving a single into right field.

Russell Martin followed with a two-run homer.

Tie game.

Troy Tulowitzki lined a single into the left-field corner before ex-Mariner Michael Saunders crushed a 2-0 sinker for a no-doubt homer to right field.

That quickly, the Mariners went from a two-run lead to a 4-2 deficit.

And the Jays weren’t done.

Far from it.

Kevin Pillar doubled to left and scored on Ezequiel Carrera’s broken-bat single to right. Six straight hits finished Iwakuma. In came Nick Vincent, and the meltdown continued.

Travis sent a soft single to right that fell just fair inside the line between Cano, racing out from second base, and right fielder Franklin Gutierrez.

Worse, Cano and Gutierrez were each slow to react to the ball as Carrera kept running — and scored all the way from first.

Travis went to second when Josh Donaldson walked before Encarnacion punched a two-run double into the right-field corner.

That made it 8-2 and knocked out Vincent as the Mariners turned to mop-up man Cody Martin, who ended the inning without further damage.

Toronto lefty J.A. Happ, the former Mariner, took the six-run lead and lasted the requisite five innings to become majors’ second 20-game winner (20-4) before handing the game to the bullpen.

There was more. Toronto also got solo homers in the sixth inning from Donaldson against Martin and in the eighth from Encarnacion against David Rollins. The Jays scored the game’s final 10 runs.

Here’s the grim updated math:

The Mariners (79-72) trail Baltimore (82-69) by three games in the race for the American League’s final wild-card berth with 11 games remaining.

That’s not all, though.

The Mariners are also 1 1/2 games behind Detroit (80-70) and one game behind Houston (80-71) in the wild-card race. Plus, the New York Yankees (78-72) are only one-half game behind the Mariners.

In short: A lot has to go right for the Mariners to end that postseason drought, starting Wednesday afternoon when they turn to Felix Hernandez in an effort to avoid a three-game sweep by the Blue Jays.

It’s not a “must” win, but…well, you know.

PLAY OF THE GAME: Nothing was the same after Mariners catcher Chris Iannetta failed to catch Jose Bautista’s foul pop in the fourth inning.

Before that, though, Toronto second baseman Devon Travis aided the Mariners by enduring a a rough third inning — although it won’t look that way in the box score.

First, Travis bobbled the ball after a nice pickup on Guillermo Heredia’s one-out grounder with a runner at first. Travis probably should been able to get one out if not two. Instead it was scored a single for Heredia.

With runners at second and third with two outs, Robinson Cano hit a grounder to Travis, who somehow kicked the ball into the right-field corner for what was scored a two-run triple.

PLUS: Robinson Cano had two hits, which is an encouraging sign after a 2-for-24 slide over the previous six games. It’s hard to see the Mariners making a late push without a productive Cano…left fielder Guillermo Heredia had two hits and also reached base when hit by a pitch.

MINUS: Let’s just limit this category to the Toronto fourth inning. Catcher Chris Iannetta has to catch the foul pop…Hisashi Iwakuma can’t unravel after the error by allowing six straight hits, including two homers…second baseman Robinson Cano and right fielder Franklin Gutierrez can’t allow a runner to score from first base on a pop-fly single.

SHORT HOPS: The crowd of 33,573 was overwhelmingly pro-Blue Jays. Just like Monday’s series opener, which drew 34,809…the Mariners are 1-4 on what was generally viewed as their most important homestand in more than a decade.