Josh Naylor Traded to the Mariners: Fantasy Baseball and Real-Life Impacts from the First Big Trade

First baseman Josh Naylor, an All-Star in 2024, has been traded from the Arizona Diamondbacks to the Seattle Mariners. While the Diamondbacks are below .500 and seem to not be to the level of a contender (especially given injuries they have sustained), the Mariners are six games above .500 and currently in wild-card position.

Houston is playing well at the top of the AL West, but Seattle is only 5.5 back (before Thursday's result) with about 60 games remaining. Every game is going to matter, so making this move a full week before the trade deadline gives the Mariners that little extra edge. A stolen game or two over the next week might make the difference.

Does this change Seattle's outlook, though? And will Naylor feel any effects in fantasy baseball? Let's look at some of the immediate questions that come from this move and where the Mariners might still be looking to improve before next Thursday.

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Did Seattle Need a First Baseman?

By wins above replacement (WAR), the Mariners rank 23rd at first base (according to Baseball-Reference). So, short answer: Yes, they needed a first baseman. They are a top-10 team by DH and pinch hitter numbers, other places Naylor might help, but he will be expected to be the everyday first baseman. This was a move made to win right now.

Luke Raley, Rowdy Tellez, and Donovan Solano have handled a lot of the first-base work. Tellez hit for some power, but he didn't do anything else and was better in that DH/bench role. Solano has a little better average (closer to .250) but doesn't hit for any power, with just three doubles and three home runs in 150 at-bats. Raley doesn't have much of either, with a low average (.220) and no power (six doubles and four home runs).

Naylor, on the other hand, is hitting .292 with 19 doubles and 11 home runs and holding his career-best on-base percentage and OPS. He is a massive step up from the incumbents at first, so on top of helping the bench and DH by pushing other guys down the depth chart, he will present something of a best-case scenario for filling his own position.

No matter what happens the rest of the season, no one can say that the Mariners didn't make a move to go for it. What they do next might matter more.

What's Next for the Mariners?

There's still a week until the trade deadline, so Seattle might not be done. The same Baseball-Reference numbers linked above rank them in the bottom 10 in pitcher WAR, starting pitcher WAR, and relief pitcher WAR. For a team that had a top-10 starting pitching staff last season, that's a disappointing fact and one the team could hope will rectify itself a bit over the last two months.

That's a dangerous assumption, though. They rank right in the middle in most pitching box stats, something that can lead to success if the offense is fantastic. Without that top-tier offense, though, it will be tough for Seattle to compete in the playoffs without major improvements from the pitching staff.

Bringing in a few bullpen pieces seems like a must, especially as those are the easiest players to find at the trade deadline. Getting a pitcher or two from the Diamondbacks could have been helpful, but those guys might not have been available. Naylor is in the last year of his contract and much more attainable if Arizona wasn't keen on paying up big to keep him.

Instead, Seattle will have to keep searching and see who they can find to improve the team. Bringing in a starting pitcher or two, even if they are more middling, can push some other guys to the bullpen and strengthen that unit also, especially when the playoff games come and pitching staffs are tightened up.

Will Naylor See Any Difference from a Fantasy Baseball Perspective?

He's still going to hit. Naylor has hit a home run about every 23 at-bats since 2022 and has knocked in 343 runs in that time. He's still a guy you want on your fantasy team.

There are a few concerns, though. The biggest one is the home ballpark he will be playing in. Baseball Savant ranks Chase Field in Arizona as the fourth-friendliest hitter's park over the past three seasons (and seventh best in 2025). The same measure sees T-Mobile Park in Seattle as the least hitter-friendly park over the past three years and the second worst this year.

That's a huge shift. Only half the games will happen at home, but Naylor is going from one of the best hitter's parks to one of the worst. That doesn't mean he's going to totally fall off, but there is a clear reason to believe that Naylor's numbers will drop over the next two months.

If someone wants to trade for Naylor in your fantasy league, it's worth considering. First base is generally the easiest position at which to find fantasy production, and if Naylor is going to feel those negative effects, you can probably find the same production over the next 60 games by streaming guys in good environments (including whoever takes over for Arizona).

Conclusion

Seattle made a clear upgrade to the roster at one of their biggest positions of need. MLB.com says that the Mariners gave up their 13th-best (LHP Brandyn Garcia) and 16th-best (RHP Ashton Izzi) prospects; that's a move I would make every time given the position they are currently in. Mariners' fans should be happy with this move and maybe ready to expect more given the needs of the pitching staff.

From a fantasy perspective, things don't look quite as rosy for Naylor over the rest of the season. Arizona has scored the fourth-most runs and Seattle the 10th most, so there will theoretically be fewer RBI chances. That said, Naylor still should be a decent fantasy guy who can make a difference on your roster, and he will provide a huge upgrade at first for the Mariners.