Boston

Smith

Smith has been one of the breakout prospects of 2026, as the former sixth-round pick has hit 22 homers while forging a 1.037 OPS while reaching Double-A. There are defensive questions with Smith — and it’s worth noting that he’s a player that turns 24 in November — but there’s enough offensive upside to suggest he could be a regular in a year or two. He’s worth monitoring in dynasty/keeper formats, at least.

In exchange for Castillo, the Mariners will acquire right-handed reliever Seranthony Dominguez, outfielder Nolan Jones, and catcher/outfield prospect Boston Smith. The Whit Sox will also take on all of Castillo’s contract, which pays him 22.75 million the next two years with an option for 2028. The Mariners have long been rumored to explore trading a starting pitcher with Emerson Hancock and George Kirby’s name bandied about, but it’s Castillo who will be heading to Chicago. The right-hander is not the same hurler he was a few years ago when he was among the very best in the sport, but he’s a solid mid-rotation option for a team that has played well above expectations with an excellent chance of reaching October. He’s more streamer than fantasy stalwart, but perhaps the change of scenery helps Castillo find the form of the previous campaigns.

Smith was the Nationals’ sixth-round pick in 2025. The 23-year-old projects as roster fodder at this stage of his career, but it’s worth noting he’s also played third base and the outfield in college; giving him some (potentially) valuable versatility.