The table is ripped from Kalk's tool database and meshes with what I've seen in the brooks pfx stuff, so I went with the one that charted an order of magnitude more pitches. What we see is a pretty decent repertoire. The fastball movement is pretty blah, but throws hard enough. Judging from the brooks graphs, his velocity increases as the inning wears on. He can get up to 95-96, but is usually in the 92-94 range. That's plus velocity. The slider doesn't look particularly good, but that's the hardest pitch to eyeball for me. The Kalk database says 16% of all sliders went for whiffs, which is well above average. The change up looks more like I'd expect a splitter to look, but it gets an average whiff rate and doesn't get too beat up when put in play. That's two plus pitches and an average one. We'd expect perhaps some struggles against lefties, but more or less domination against RHB. Remember the chart from the Dan Hudson BUO?
The suggestion is that an average fastball/slider combo will tear through a right handed lineup (this is actually my guess for the Sox woes against pitchers they haven't faced). Assuming Snell's stuff has been more or less the same throughout his career, what's he done with it?
Career vs. RHB: .265/.327/.425, .320 BABIP, ~ .333 wOBA against.
Career vs. LHB: .293/.385/.483, .325 BABIP, ~ .386 wOBA against.
Well, not that much it turns out. Average righties stay average. Average lefties turn into Jim Thome. He's given up about 5 runs per 9 innings pitched over his career and it's become worse lately as his control has imploded. Lookout Landing (ht: WU!) has speculated that there are mechanical issues, especially with his foot landing. I'm not entirely sure there, since the slo-mo may be making it look worse than it is. There's a lot of shock that goes into a pitching motion in the first place. Still, it doesn't look great.
The brooks pfx was somewhat suggestive, but they really aren't calibrated to doing that kind of analysis--the data hasn't been corrected. In any case, the plots for each pitch type were very spread out and every individual pitch seemed to be doing something somewhat different than its similarly categorized brethren. This may be a visual rendering of poor control: if you don't have repeatable mechanics, it's likely that your pitches will vary more than a pitcher with repeatable mechanics. In any case, we can just point to Snell's walk rate, which for the last two seasons has been well above his career 4.12 BB/9 rate. In addition, he's had an elevated BABIP for his career and he's going to make his 125th MLB start tonight. Bad pitches get hit hard and are probably harder to defend. His lack of command is hurting him in more ways than one.
Nonetheless, he's obviously talented. It's why the M's acquired him when they had the chance. Frankly, it's too bad the Sox weren't in the running for his services. To echo some SSSers before the trade deadline, I'd be very interested to see what Coop could do with him.