Mariners vs. Astros Prediction, Odds, Probable Pitchers and Prop Bets for Sunday, Sept. 21

Betting odds, pick and prediction for the Seattle Mariners-Houston Astros matchup on Sunday night.

Seattle Mariners starting pitcher Logan Gilbert is a solid prop target on Sunday. / Joe Nicholson-Imagn Images

The Houston Astros are aiming to get back in the AL West race on Sunday, as they are two games back of the Seattle Mariners and now tied with the Cleveland Guardians for the final wild card spot in the AL.

Houston’s season is hanging in the balance, and it’s set as a home underdog on Sunday against Seattle.

Jason Alexander has led the Astros to a 12-4 record in his outings this season, but can he get them a win on Sunday Night Baseball?

It won’t be easy against Seattle’s Logan Gilbert, who is coming into this matchup with a 3.53 ERA. 

Here’s a breakdown of the odds, players to watch and my prediction for Sunday’s matchup. 

Mariners vs. Astros Odds, Run Line and Total

Odds via DraftKings Sportsbook

Run Line

• Mariners -1.5 (+119)
• Astros +1.5 (-145)

Moneyline

• Mariners: -142
• Astros: +117

Total

• 8 (Over -116/Under -104)

Mariners vs. Astros Probable Pitchers

• Seattle: Logan Gilbert (5-6, 3.53 ERA)
• Houston: Jason Alexander (4-1, 4.04 ERA)

Mariners vs. Astros How to Watch

• Date: Sunday, Sept. 21
• Time: 7:00 p.m. EST
• Venue: Daikin Park
• How to Watch (TV): ESPN
• Mariners record: 86-69
• Astros record: 84-71

Mariners vs. Astros Best MLB Prop Bets

Mariners Best MLB Prop Bet

• Logan Gilbert UNDER 17.5 Outs Recorded (-108)

Logan Gilbert has recorded 18 or more outs in just seven of his 21 starts this season, and now he’s facing a Houston team on the road on Sunday.

Gilbert has only thrown 100 pitches once in the 2025 season, so it’s unlikely that he works deep into this game even if he does pitch well. In his outings this month, Gilbert has recorded 15, 21 and 14 outs. 

I’ll fade him from completing six innings on Sunday. 

Mariners vs. Astros Prediction and Pick

I’m going to trust the two starting pitchers in the first few innings in this matchup.

Gilbert has allowed three or fewer runs in eight of his nine starts since Aug. 1, and he’s posted a 3.78 ERA over that stretch. He also allowed just one hit in his lone matchup against Houston in 2025. 

Jason Alexander has also been solid, leading Houston to a win in each of his last nine outings – making it tough to pick a side in this game.

Over that nine-game stretch, Alexander has a 2.32 ERA, allowing three or fewer runs in every one of those outings.

I think it’s a no-brainer to take the UNDER in the early innings with how hot these starters have been entering this matchup. 

Pick: First 5 Innings UNDER 4.5 (-120 at DraftKings)

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Odds refresh periodically and are subject to change.

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