Astros vs. Diamondbacks Prediction, Odds, Probable Pitchers and Prop Bets for Monday, July 21

Iain MacMillan | Jul 21, 2025

The Diamondbacks are home favorites against the Astros on Monday night. / Rick Scuteri-Imagn Images

The Arizona Diamondbacks enter this week's series against the Houston Astros at .500, meaning they aren't out of the mix yet, but they need to put together a great stretch of games sooner rather than later if they want to get back in the playoff mix.

Meanwhile, the Houston Astros hold a four game lead over the Seattle Mariners in the AL West.

Let's dive into the odds and my best bets for this interleague showdown.

Astros vs. Diamondbacks Odds, Run Line, and Total

Odds via BetMGM Sportsbook

Run Line

• Astros +1.5 (-175)
• Diamondbacks -1.5 (+145)

Moneyline

• Astros +120
• Diamondbacks -145

Total

• Over 9 (-102)
• Under 9 (-118)

Astros vs. Diamondbacks Probable Pitchers

• Houston: Colton Gordon, LHP (3-2, 4.67 ERA)
• Arizona: Zac Gallen, RHP (7-10, 5.40 ERA)

Astros vs. Diamondbacks How to Watch

• Date: Monday, July 21
• Time: 9:40 p.m. ET
• Venue: Chase Field
• How to Watch (TV): Space City Home Network, Dbacks.TV
• Astros Record: 57-42
• Diamondbacks Record: 50-50

Astros vs. Diamondbacks Best MLB Prop Bet

• Colton Gordon UNDER 3.5 Strikeouts (-115) via BetMGM

In today's edition of Painting Corners, I broke down why I'm betting the UNDER on Colton Gordon's strikeout total:

In my first pick today, I wrote about how the Blue Jays have the best strikeout rate against left-handed pitchers. The team that's in second place, right behind them, is the Arizona Diamondbacks, who have struck out on just 18.6% of plate appearances against lefty pitchers.

Tonight, they'll take on a lefty starter in Colton Gordon of the Houston Astros, who has failed to reach 4+ strikeouts in three straight starts. Let's fade him again against these Diamondback bats.

Astros vs. Diamondbacks Prediction and Pick

I'm going to sit back and root for runs in this interleague matchup. Neither pitcher is anything to write home about, with Gordon (4.67 ERA) facing off against Zac Gallen (5.40 ERA). Gallen has allowed 5+ earned runs in three of his last five starts and has shown some significant regression this MLB season.

Offensively, both teams enter this game hot. Over the past 30 days, the Diamondbacks and Astros rank eighth and ninth in OPS. Neither lineup should have issue putting runs on the board against the opposing starting pitchers.

Let's bet the OVER tonight.

Pick: OVER 9 (-102) via BetMGM

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