Reds vs. Mariners Prediction, Odds, Probable Pitchers, Prop Bets for Tuesday, July 21

The Seattle Mariners are back in first place in the AL West after an 8-0 win over the Cincinnati Reds on Monday, and they’re set as slight favorites to take the second game of this three-game set on Tuesday night.

Cincinnati has fallen to last place in the NL Central after a strong start to the season, and it has a lot of work to do in the second half if it wants a shot at a wild card spot. The Reds have fallen to 20th in MLB in OPS and 24th in team ERA, a bad combination if you’re attempting to make the playoffs.

Meanwhile, the Mariners have a division lead even though they’re just one game over .500 this season. Seattle hasn’t exactly been dominant, but a weak AL West should allow it an opportunity to make the playoffs.

On Tuesday, Seattle veteran Luis Castillo (4.93 ERA) is set to make his 16th start of the season after working a few outings from the bullpen in May and June. Seattle is still favored with him on the mound, even against Cincinnati ace Chase Burns, who has a 2.54 ERA across 18 appearances in 2026.

Let’s take a look at the odds, a player prop to bet and my prediction for Game 2 of this series.

Reds vs. Mariners Odds, Run Line and Total

Odds via DraftKings Sportsbook

Run Line

Reds -1.5 (+162)
Mariners +1.5 (-197)

Moneyline

Reds: -106
Mariners: -114

Total

7.5 (Over -108/Under -112)

Reds vs. Mariners Probable Pitchers

Cincinnati: Chase Burns (11-1, 2.54 ERA)
Seattle: Luis Castillo (3-8, 4.93 ERA)

Reds vs. Mariners How to Watch

Date: Tuesday, July 21
Time: 9:40 p.m. EST
Venue: T-Mobile Park
How to Watch (TV): Mariners.TV/Reds.TV
Reds record: 45-54
Mariners record: 51-50

Reds vs. Mariners Best MLB Prop Bets

Reds Best MLB Prop Bet

Elly De La Cruz to Hit a Home Run (+368)

In today’s edition of Daily Dinger – SI Betting’s best home run picks column – I broke down why Cruz is a solid bet to stay hot against Seattle:

Over the last 28 days (22 games), Cincinnati Reds star Elly De La Cruz has homered just four times, but I’m buying him to hit his 17th long ball of the 2026 season on Tuesday night.

The Reds are set to take on the Seattle Mariners and right-hander Luis Castillo, who enters this start with a 4.93 ERA and 12 home runs allowed in 18 appearances (15 starts). Castillo has not been his usual All-Star self in 2026, and he allowed three home runs in his final start before the break.

That’s a good sign for De La Cruz, who has a .773 OPS and eight home runs against right-handed pitching this season. The Reds shortstop has been better against left-handed pitching, but he’s been red hot over his last 10 games, hitting .325 with three home runs. Since the break, he’s hitting .375 with one homer in four games.

Reds vs. Mariners Prediction and Pick

Cincinnati has been dominant when Burns is on the mound this season, going 14-4 in his 18 starts.

The Reds ace has been one of the best pitchers in MLB, posting an expected ERA in the 83rd percentile and an expected BAA in the 87th percentile. He’s struck out 118 batters in 102.2 innings of work, allowing two or fewer runs in 15 of his 18 outings.

Burns has a massive edge over Castillo, who has allowed at least four runs in nine of his outings this season. The veteran right-hander ranks in the 33rd percentile in expected ERA, and the Mariners are just 5-10 when he starts.

I’m going to trust Burns to shut things down in the early innings, and the Reds have found a way to score enough to win his last six outings.

Pick: Reds Moneyline (-106 at DraftKings)

Odds refresh periodically and are subject to change.

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