A’s vs. Mariners Prediction, Odds, Probable Pitchers, Prop Bets for Tuesday, April 21

The Seattle Mariners dropped their series opener with the A’s on Monday night, but they find themselves as favorites in Tuesday’s Game 2.

Seattle has righty Luis Castillo on the mound, and he’s looking to turn around a slow start to the 2026 season.

Castillo has a 5.40 ERA, and he’s allowed at least four runs in each of his starts this month. The A’s will counter with lefty Jacob Lopez (6.38 ERA), who may have an advantage in this matchup based on some of his advanced stats.

Lopez ranks in the 81st percentile in expected batting average against and the 87th percentile in hard-hit percentage.

I’m eyeing a player prop for the Mariners, as well as a pick for the outcome of this game.

Here’s a look at the odds for Tuesday’s AL West showdown.

A’s vs. Mariners Odds, Run Line and Total

Odds via DraftKings Sportsbook

Run Line

A’s +1.5 (-156)
Mariners -1.5 (+129)

Moneyline

A’s: +139
Mariners: -168

Total

7.5 (Over -112/Under -108)

A’s vs. Mariners Probable Pitchers

A’s: Jacob Lopez (1-1, 6.38 ERA)
Mariners: Luis Castillo (0-1, 5.40 ERA)

A’s vs. Mariners How to Watch

Date: Tuesday, April 21
Time: 9:40 p.m. EST
Venue: T-Mobile Park
How to Watch (TV): Mariners.TV, NBC Sports California
A’s record: 12-11
Mariners record: 10-14

A’s vs. Mariners Best MLB Prop Bets

Mariners Best MLB Prop Bet

Cal Raleigh to Hit a Home Run (+310)

Earlier today, I shared in SI Betting’s best home run picks column – Daily Dinger – why Raleigh is worth a look to get out of his slump:

Raleigh is off to an extremely slow start in the 2026 season, hitting .161 with just three home runs, but I think he’s worth a look against the A’s on Tuesday.

The A’s have a shaky bullpen (4.50 ERA, 10 homers allowed in 2026), and Lopez has not fared well against Raleigh in the past. The Seattle catcher is 2-for-2 with two homers in his career vs. Lopez – a 100 percent hit rate when it comes to home runs!

Now, I’m not expecting Raleigh to homer every time he faces Lopez, but the A’s righty has allowed three homers in four starts this season, posting a 6.38 ERA. Last season, Lopez allowed 15 homers in 17 starts (21 appearances), so it wouldn’t be a surprise if he averages around one homer allowed per game in the 2026 campaign.

Raleigh’s recent numbers aren’t inspiring, so I’d understand bettors staying away from this prop. However, the Mariners catcher has too much power for me to fade him in such a favorable matchup.

A’s vs. Mariners Prediction and Pick

I’m buying the A’s to at least cover the run line in Game 2 of this early-week series, especially since Seattle has struggled on offense.

The Mariners are in the bottom 10 in MLB in runs scored and OPS this season, and that’s led to a 9-15 record on the run line.

The A’s are 13-10 on the run line this season, and Lopez may be undervalued based on his ERA heading into this game. As I mentioned earlier in this story, Lopez has some impressive underlying metrics, and his expected ERA of 3.99 is much better than his actual mark.

That’s a sign that he’s due for some positive regression this season.

Meanwhile, Castillo has given up 26 hits in 18.1 innings of work in 2026. He ranks in the seventh percentile in expected batting average against (.307) and the 19th percentile in expected ERA (5.60).

I’m fading him until he shows his All-Star form in the 2026 season.

Pick: A’s +1.5 (-156 at DraftKings)

Odds refresh periodically and are subject to change.

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