Fantasy Baseball: Luis Castillo helped by trade to Mariners, but George Kirby may lose his spot

The Mariners kicked off what could be a big weekend of deadline dealing by acquiring Luis Castillo from the Reds on Friday. Castillo was arguably the best starting pitcher rumored to be on the move, and with another year of control beyond this one, it took a haul to get him — namely, a four-player package headlined by shortstop prospects Noelvi Marte and Edwin Arroyo.

First, what it means for Castillo. Not only is he going from a non-contender to a contender but he’ll also enjoy a significant park upgrade, going from one of the hitter-friendliest to one of the pitcher-friendliest. You could argue venue makes less of a difference for a pitcher who consistently ranks among the top five in ground-ball rate, but Castillo has faded his sinker in recent weeks for more four-seamers — a change that has yielded positive results overall. (He has gone from 7.8 K/9 before May 31, when he first ramped up the four-seam use, to 10.1 K/9.) Still, it has made him less ground ball-oriented, and relatedly, he has a 3.64 ERA at home this year compared to 2.09 on the road.

So what does that mean, practically speaking? Probably that we should treat Castillo like a top-20 starting pitcher in Fantasy again. His ERA is already a career best 2.86, and I’d now bet on him keeping it below 3.00. I’d also bet on him having better than a .500 record (4-4) moving forward.

The Mariners benefit from this acquisition in an indirect way, too. They now have the flexibility to manage the workloads of sophomore Logan Gilbert and rookie George Kirby, whose innings are beginning to creep up on them. I recently highlighted both in my 18 pitchers with workload concerns. Kirby is the more concerning of the two, having already eclipsed his previous high by about 25, so we may even see him shut down for a period. As long as everyone is healthy, the Mariners would have to go six-man to keep him in the rotation.

As for the prospects, Marte and Arroyo, both were in my midseason top 50, with Marte checking in at No. 19. That’s a drop from my preseason top 100, actually, but it could have been worse after he showed up out of shape and initially struggled both offensively and defensively. A big July has kept his stock from tumbling further, but it’s pretty obvious at this point that he’s not sticking at shortstop. It may be why the Reds insisted on Arroyo as well. He’s one of this year’s breakout prospects, batting .316 with 13 homers, 21 steals and an .899 OPS at Low-A. The California League is a favorable place to hit, but when you consider he’s doing it at age 18, it’s hard not to get excited about his upside.

Of course, the Reds already had two notable shortstop prospects, the big league-ready Jose Barrero and the surging 6-foot-7 behemoth Elly De La Cruz. It’ll be fun to see how they make all the pieces fit in the years to come.