Similar to Crawford, there is very little reason for the Mariners to trade Justus Sheffield and his (hopefully) 4-years of service time at this point in time. Sheffield flashed both swing-and-miss stuff and the ability to generate groundballs in his first extended look at the big leagues.
Lefties like that aren’t easy to find in baseball and even fewer exist with the athleticism to match Sheffield. His slider is one of the best in the game and the changeup has really improved since coming to Seattle. The pitch he needs more value from is the fastball and Sheffield has made strides there, switching to a 2-seam grip as his primary grip.
Even if Sheffield fails in the rotation, it is hard not to see him be a success in the bullpen where his plus slider will play even better when he doesn’t have to worry about getting through the lineup multiple times.
Even if Sheffield posts solid, #4 starter numbers in 2020, there still isn’t much reason to trade him this winter, unless the Mariners are receiving a better player who fits their timeline as well as Sheffield does.
The same could also be said for Justin Dunn who has a similar floor as Sheffield with a slightly higher ceiling. Neither pitcher is likely to be dealt this summer or this winter and will more than likely be permanent members of the pitching staff come 2021.