The Mountain West tournament is three weeks away in Las Vegas.

That’s when the conference’s automatic bid to the NCAA Tournament will be decided.

That might be San Diego State’s only way to get there.

The Aztecs suffered the loss that a team sitting smack on the bubble absolutely can’t afford at this juncture of the season, 73-63 against Grand Canyon at Viejas Arena on Tuesday night.

There’s probably still a path to an at-large bid, but it likely would require sweeping the final five regular-season games — four of which are against the conference’s upper echelon, including one at New Mexico and another at Boise State — and then winning a couple more in Las Vegas.

But it’s a situation the Aztecs (18-7, 12-3) never dreamed they’d be in back in November, when they opened the season with eight returnees off an NCAA Tournament team, three preseason all-conference players and two projected NBA draft picks.

The season seemed to be trending up after a rocky start … until Tuesday night.

SDSU’s all-time record against Grand Canyon: 1-6.

SDSU’s conference record this season against everyone but GCU: 12-1.

The first meeting last month was decided on a controversial call with 1.8 seconds left that gave the Antelopes the winning free throws after BJ Davis had missed the front end of a one-and-one at the other end. No favorable whistles were necessary this time.

The Antelopes (17-9, 10-5) led by nine in the first half, trailed by four at the half, led by 12 in the second half and then hung on for dear life as the Aztecs came roaring back again. It was a five-point game with four minutes left, but back-to-back turnovers stalled the comeback.

The dagger was a follow dunk by 7-foot-1 Efe Demirel with 1:17 to go after Magoon Gwath had swatted Makaih Williams’ layup off the glass.

Reese Dixon-Waters (six points, 2 of 9 shooting) missed at the other end, the Antelopes made a pair of free throws, and that was that.

Gwath played his second game after missing six and wasn’t nearly as effective as in the 14-point win against Nevada on Saturday, when he had 13 points and five blocks in 14 minutes.

His line Tuesday: four points, 2 of 9 shooting, 0 of 3 beyond the arc in 20 minutes. (And no points in the game’s first 37 minutes.)

Miles Byrd got going after a slow start, not scoring for the game’s first 12½ minutes and finishing with 18 points on 6 of 10 shooting. No one else was in double figures.

The Antelopes had three: UNLV transfer Jaden Henley, who won here last year with the Rebels, with 21, Demirel with 14 and Williams with 11.

It was pretty much what you’d expect when two top 30 defenses get together with an officiating crew that decided to let ’em play.

The score six minutes into the game: 6-6.

The Aztecs took nearly nine minutes to crack double figures and trailed by as many as nine. They were stuck on 19 and trailed by seven when 6-foot-6 freshman Tae Simmons subbed in with 4:35 left in the first half.

Not as a small forward or power forward, but as the center.

His defensive assignment: Demirel, the 7-1, 240-pound, 21-year-old Turkish “freshman.”

It was largely by necessity, with SDSU centers Miles Heide and Pharaoh Compton both on the bench with two fouls (as was BJ Davis). It was also by design, though, since the Aztecs made their late run from down 12 at Grand Canyon last month with Simmons at the 5 paired with four guards.

And it worked again.

The Aztecs closed the half on a 14-3 run, with Simmons accounting for six of the points – two dunks and a pair of free throws after snatching an offensive rebound amid the tall trees.

It gave the Aztecs an improbable 33-29 margin at the break despite shooting 12 of 30 overall and 1 of 7 beyond the arc while allowing the Antelopes to shoot 4 of 9 from deep. Time with the lead: 2:09.

But the halftime break cooled all those positive vibes, and the second half started much the same way as the first. The Aztecs missed 12 of their first 15 shots and soon found themselves in a 12-point hole that they ultimately couldn’t climb out of.

“Six and one,” purple-clad GCU fans chanted as the final seconds ticked off.

Notable

Next up: Saturday at Colorado State (4 p.m., CBS Sports Network) … Grand Canyon entered the night with a 5-1 record against SDSU. Only Washington State (6-0) has a higher winning percentage against the Aztecs in at least that many games … Antelopes coach Byrce Drew won his 300th game as a Division I head coach on Saturday against San Jose State. They have come at Grand Canyon (126), Valparaiso (124) and Vanderbilt (40) … The officials reversed several calls, one after initiating a review