Trent Thornton's flagging velocity has to be a concern for Blue Jays

The Toronto Blue Jays 2019 season has been full of surprises from Bo Bichette’s historic start, to the Vladimir Guerrero Jr.’s Home Run Derby brilliance, to the head-scratching Aaron Sanchez trade.

Amidst all of the strangeness, one of the weirder occurrences has to be Trent Thornton ascending to rank of grizzled veteran in the team’s rotation. The right-hander had started precisely 0 MLB games coming into the season and was supposed to open the year at Triple-A and wait his turn to break through.

With Sanchez and Marcus Stroman traded - plus injuries to Clayton Richard, Clay Buchholz, Matt Shoemaker, and Ryan Borucki - Thornton hasn’t just started more games than any other Blue Jays, his 25 starts are more than anyone else in the rotation has in their career.

Generally speaking, the 25-year-old has accounted for himself well enough. His ERA of 5.30 isn’t pretty but his 1.3 WAR ranks seventh among rookie starters, and his elite spin rates continue to speak to a ceiling he hasn’t yet reached.

However, what the right-hander has been showing lately is on the concerning side. Since the all-star break - admittedly a bit of arbitrary starting point - he’s got an ERA of 6.92 and FIP of 6.37 and a strikeout rate that’s plunged from 23.2 percent to 13.7. Considering Thornton’s trademark is his high-spin breaking balls, a failure to miss bats of that magnitude is rather jarring.

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We are talking about just 26 innings here, but there’s more than small-sample size weirdness at work here. In fact, there’s a very simple and concrete explanation for what’s happening - Thornton’s velocity is falling off a cliff.

Here’s a chart that shows his average fastball velocity against his strikeout rate:

It’s not a perfect 1:1 relationship, but it seems clear that a lack of juice is a real issue. Earlier in the year, Thornton was averaging 94 mph, or thereabouts, with regularity. In his last three starts he’s been at 91.8 mph, 92.0 mph and 92.1 mph. That takes away his fastball as a swing-and-miss weapon.

Part of what makes Thornton so good when he’s rolling is that opposing hitters have to account for high fastballs and breaking balls below the zone. When they can key in on only one, he’s far less dangerous.

It seems like an oversimplification to pin all of Thornton’s woes on a falling strikeout rate, but the knock-on effect is fairly evident. With far more at-bats where hitters are able to put their bat on the ball, they’ve unsurprisingly done quite a bit more damage.

For example, Thornton’s HR/FB rate is about the same in the second half (15%) as the first (13%), but his HR/9 has jumped from 1.33 to 2.08. Even if the dangerous contact he allows is similar on a rate basis, he’s putting himself in position to get hurt more often.

The question is where the Blue Jays go from here. They aren’t really in a position to pass up whatever innings Thornton can give them if he’s healthy. He also doesn’t seem like a natural candidate to be shut down because he’s only pitched 120.2 innings this season compared to 140 next year. If anything, the club will be hoping to push his workload forward.

Perhaps the velocity dip is an indication of a minor injury, or fatigue that puts him at a greater risk of getting injured. If that’s the case the right-hander shouldn’t be pitching.

Assuming that he’s physically fine, the Blue Jays are probably best served by continuing to run him out there so he can fill a spot in the rotation and build up his innings. That course of action is logical, but it may not be pretty from here on out.

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