Merryweather looks like he could benefit tremendously from altering his pitch usage a bit. He would likely benefit from lowering his fastball usage as he has been torched the last few seasons. The statcast numbers don't really suggest he's been unlucky with the pitch either as he's likely benefited from the deadened baseball. His xWOBA of .625 against fastballs is actually worse than the staggeringly bad .543 WOBA. Throwing this pitch over half the time to the tune of 52.1% makes no sense when it appears as though the opposition is likely sitting on this pitch.
When you look at the results against his other pitches you see a completely different picture of dominance emerge;
Slider: .191 WOBA/.148 xWOBA, 46.2% whiff rate, 23.7% usage. This is a money pitch, he should likely throw this half of the time until hitters show they can touch it
Changeup: 0.00 WOBA/.087 xWOBA, -24 degree launch angle, 16.7% whiff rate, 14.2% usage. Another money pitch, not a lot of whiffs but generating an insane 72.1 MPH exit velocity. Maybe it doesn't play up as much if he reduces fastball rate too much.
Curveball: 0.00 WOBA, .142 xWOBA, 28.6% whiff rate, 72.4 MPH exit velocity, 10% usage. Yet another potentially elite secondary pitch.
I think with a drop of 4 seam fastball use to something of the 30-40% range will likely do Merryweather a world of good as up to this point the opposition has been totally shut down by his secondary pitches.