Reason No. 1: Stats show that Marco’s offspeed pitches are very hittable
Marco Gonzales’ sinker and cutter were nearly untouchable according to Baseball Savant. In 2020, opposing batters had a batting average of .195 against his sinker. Against his cutter, batters only hit .188. These are extremely impressive numbers, and those two pitches accounted for roughly 70% of all the pitches he threw.
The other 30% of pitches include his changeup and curveball which are my sources of worry. In 2020, opposing batters had a .310 average against Marco’s changeup. As far as his curveball, the batters posted a .262 average. The easy answer might be to throw those pitches less or throw a different type of curveball or changeup, but that is a tall task.
What makes Gonzales so effective is his ability to change the timing of hitters by mixing in a variety of pitches so he still needs his curveball and changeup despite their low effectiveness. His curveball had the highest K% out of his pitches in 2020, so it is not entirely a bad pitch to finish off batters. As a big leaguer, it can be hard to re-learn a pitch or change the grip and have it be immediately effective so that is not a viable solution either.
2019 was a similar story where batters hit .324 against his curveball. In 2020, batters slugged .571 against his changeup and slugged .405 against his curveball which is also a big number. The highest hard-hit % came off his curveball at 41.4%, compared to his sinker which was 32.2%.
There are some other statistics that are concerning in terms of how his stuff plays out. Part of these numbers are a result of his pitching style but are important to still consider. Marco’s whiff percentage is in the 9th percentile in the 2020 MLB rankings (the higher the better). His barrel percentage is in the 31st percentile. Marco’s fastball spin rate is in the 25th percentile, and his curveball spin rate is in the 23rd percentile.
Gonzales was still a very effective pitcher in 2020 and is trending in the right direction. But I do not think the numbers I listed above will improve dramatically over the next couple of years. Thus, I think Marco is not the BEST answer to lead the rotation. And I put an emphasis on best because he still would be a good ace in the MLB, but my point is the Seattle Mariners could do better. Moving on to reason No. 2.