The Arizona Diamondbacks are starting to find their stride as we approach a critical part of the season before the trade deadline, and they’ll aim for a fifth straight win here against the Houston Astros which could influence them to keep some of their best players for the stretch run.
We’ll explain in our Astros vs. Diamondbacks predictions why Houston’s starting pitcher will be too difficult to trust in a nightmare matchup against a power-hitting club.
Without further ado, let’s hand out some MLB picks for Monday, July 21.
Astros vs Diamondbacks prediction
My Astros vs Diamondbacks best bet: Diamondbacks moneyline (-145 at Caesars)
The Houston Astros have had an incredible knack for developing pop-up starting pitchers in the last few years, and Colton Gordon appeared to be their latest success story earlier in the year before hitting a bit of a snag in the last month and a half.
Gordon’s Expected Batting Average jumped to .305 in June, and has remained above .300 this month. He’s also seen his Expected Slugging continue to rise, hitting an unsightly .515 in July, and there aren’t a ton of easy things to point to as to why. The book may be out on the 26-year-old rookie left-hander, and while he’s never been a huge strikeout arm at this level, the whiffs have fallen off a cliff this month, which has put the onus on Gordon to gather outs on contact.
He's pitched predominantly to fly balls, and while his outfield has been stellar all year, his home park is unkind to fly-ballers. He’s only been slightly better on the road, however, and he’s averaged one homer allowed in either split.
This is a tough one for Gordon against an Arizona Diamondbacks team which has feasted on lefties and fly-ball pitchers all the same. Arizona is sporting a swollen .215 Isolated Power over the last two weeks, and while its high walk rate won’t necessarily help against a strike-thrower like Gordon, its persistent strikeout issues shouldn’t play a factor here.
On the flip side, Zac Gallen has been struggling to find his previous form. Pitching predominantly to ground balls, he’s failed to get much help from his infield — ranked 21st in Outs Above Average — but he hasn’t exactly made the task at hand an easy one with a 46.7% hard-hit rate this season and .262 xBA. That number peaked at .296 last month, and while it’s come down a bit in July, it remains .274 for the month.
Gallen’s whiff and strikeout rates have at least begun to climb, which should mitigate some of the issues on contact, and Houston’s offense has run cold over the last 14 days due in large part to its exorbitant 25.4% strikeout rate.
I think there are enough signs here that Gallen is at least making modest strides on the mound, while Gordon is heading sharply in the wrong direction. With the Diamondbacks slugging up a storm, I’ve got to take the home favorites.
Caesars QuickPick: Our Astros vs Diamondbacks same-game parlay (SGP)
Diamondbacks moneyline
Zac Gallen 6+ strikeouts
Geraldo Perdomo Over 1.5 hits/runs/RBI
The trends here for Gallen are encouraging. His whiff rate is back up to 28.1% this month, matching where it was in April, and that’s led to a beefy 30.1% strikeout rate, which has seen him twice hit nine punchouts in three outings this month.
With Houston experiencing some significant contact issues at the plate, getting a bat on the ball at the fifth-lowest clip over the last 14 days, this would seem to be the right spot to back the right-hander for a big night.
I’m also a huge fan of Geraldo Perdomo, who is hitting .259 against fly-ball pitchers with a team-high 38 RBIs and has also mashed lefties to the tune of a .314 average with 19 RBIs in that split – good for second. He’s now accumulated six hits in his last three games with seven RBIs and two runs to boot.
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Astros vs Diamondbacks odds
Astros vs Diamondbacks live odds
Astros vs Diamondbacks opening odds
- Moneyline: Houston +130 | Arizona -154
- Run line: Houston +1.5 (-161) | Arizona -1.5 (+135)
- Over/Under: Over 9 | Under 9
Odds courtesy of Caesars.
Astros vs Diamondbacks trend
The Houston Astros have hit the Game Total Over in 9 of their last 13 games (+5.80 Units / 41% ROI). Find more MLB betting trends for Astros vs. Diamondbacks.
How to watch Astros vs Diamondbacks and game info
| Location | Chase Field, Phoenix, AZ |
| Date | Monday, 7-21-2025 |
| First pitch | 9:40 p.m. ET |
| TV | SCHN, ARID |
| Astros starting pitcher | Colton Gordon (3-2, 4.67 ERA) |
| Diamondbacks starting pitcher | Zac Gallen (7-10, 5.40 ERA) |
Astros vs Diamondbacks latest injuries
Astros vs Diamondbacks weather
Odds are correct at the time of publishing and are subject to change.
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