Is Trent Thornton Good?

The Cubs have a stealth relief ace. Trent Thornton has a 2.38 ERA in 40 appearances. He's saved three games, despite only being called upon for that duty in emergencies. He has a 1.3 Win Probability Added, so even adjusting for leverage and situation, he's been solidly above-average. This season, only two Cubs pitchers (Ben Brown and Javier Assad) have a higher WPA than Thornton's. 

The Cubs also have a ghastly liability. One of their most important middle relievers is unable to avoid running into barrels, walks almost 10% of opposing batters, and has fanned only 14.9% of them. He doesn't miss bats; he doesn't fill up the strike zone. When opponents get the ball in the air, they usually hit it hard, so he's vulnerable to homers. He's only given up four of them this season, but that number could easily be twice as high. A bunch of fly balls have come down just on the right side of deep fences.

Which player is real: the stud or the dud? It's hard to tell for sure. Thornton doesn't throw so many strikes that he should be able to get away with the ball being in play as often as he does. Yet, there's something real in the way he's doing it—just enough to give one pause before casting him aside as an option for a team bound for the postseason.

Here's Thornton's spin profile by pitch type this season. The left-hand array shows the spin direction of the ball out of his hand; those are the directions the batter expects the ball to move based on the way Thornton releases the ball. The right-hand graphic, however, shows the actual direction of movement.

Most pitchers get at least a little unexpected movement on one of their pitches, utilizing seam effects to change the way air moves over the ball. Hardly anyone gets such significant differences on so many offerings, though. Most of Thornton's pitches achieve what's called spin mirroring, where if the hitter reads the spin axis, they'll be unable to tell whether he's throwing a fastball (his sinker and four-seamer spin almost identically out of the hand) or a breaking ball (his curve and sweeper spin similarly, too, and almost precisely the opposite direction of the fastballs). The curve moves mostly as you'd expect, but the hitter can't fully commit to it, because of that spin mirroring. The sweeper, though, has much more lift and glove-side movement than the spin direction would imply. 

Thornton's four-seamer is similarly faithful to its spin direction: it's a cut-ride offering. His sinker, however, runs more to the arm side than the hitter would expect. His cutter, meanwhile, mostly has backspin out of his hand, but veers to the glove side. Because Thornton neither throws especially hard nor achieves exceptional movement, he doesn't rack up many swings and misses, and that does mean running into barrels sometimes. It's not quite right to say that it's hard to square him up. It's more correct, in a strange way, to say that it's easy to mishit him. He can, when he executes and the hitter isn't in a position to outguess him, induce a lot of low-value contact, even if that's not precisely soft contact. He runs a high ground ball rate, by crawling out to the end of opponents' bats and up the handle. He gets a fair number of fly balls that are well-struck, but which die on or around the warning track.

That's not the profile you'd sketch for a high-value, high-leverage October relief arm. I have Thornton high on the list of arms the Cubs should try to replace, as some of their currently injured relievers get healthy in the coming weeks. So far, though, he's been quite effective, and it's not all smoke and mirrors—or, more correctly, it's mostly smoke and mirrors, but not necessarily the kind that lose all effect once they're spotted.

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