Is Cubs Reliever Trent Thornton Just Getting Lucky? Or is He the Right Person at Just the Right Time?

If Tuesday night’s victory over the Baltimore Orioles was any indication, the Cubs might start looking to Trent Thornton to pitch in higher-leverage opportunities going forward. The right-hander notched his second save of the season with a scoreless ninth inning. This was in a game that was after an off day, which is to say that the bullpen was rested. Craig Counsell had his pick of the litter, and he picked Thornton. 

That is quite the leap for a guy who started the season in Iowa and wasn’t called up to the big-league club until early May. It’s also quite the leap for me, because I have a small bias: I have a really hard time believing in relievers who don’t rack up big strikeout numbers. This makes Thornton and his minuscule 12.5% strikeout rate hard for me to embrace. No qualified pitcher has a lower strikeout rate. Combine that with a walk rate a tick above league average, and you have what looks to be a frustrating bullpen arm. But should I trust Thornton more? Counsell is certainly starting to, and he knows way more than I do.

The thing is, Thornton has gotten results. His 2.63 ERA out of the bullpen is one of the best on the team. However, any advanced metric would suggest that is entirely smoke and mirrors. He has a 5.03 xERA and a 5.28 FIP. His .171 BABIP (batting average on balls in play) is the sixth-lowest among pitchers with at least 20 innings pitched, according to FanGraphs. For context, league-average BABIP is .288 so far this season. 

So, he doesn’t strike a lot of people, but he gets them out when they put the ball in play. What’s the big deal? Maintaining a BABIP that low isn’t usually a product of any particular skill; it’s luck. Put enough balls in play and eventually some of them will find a hole. That being said, there’s still a scale to BABIP. Some pitchers run higher ones, and others lower ones. Can’t Thornton just be one of the guys that runs a low one?

The answer is yes, though probably not as low as .171. Here's what Thornton has going for him: entering Wednesday’s action, he is posting a 50.6% ground ball rate and just a 13.9% line drive rate. By giving up a lot of ground balls, Thornton avoids giving up extra-base hits, and by allowing so few line drives, he avoids the type of contact that generally leads to a high batting average. 

So why does his FIP think he is getting so lucky? Even though he is keeping the ball on the ground, the former Tar Heel is still getting lucky when you consider how few of those ground balls manage find the outfield grass:

He’s getting better results on line drives and fly balls than expected, though not egregiously so. It’s the ground balls that really stick out. In fact, among 411 pitchers who have recorded at least 25 ground balls this year, only one has a more significant gap between his batting average allowed and his expected batting average than Thornton, per Baseball Savant.

If you sort through all of Thornton’s outs recorded on the ground, you’ll definitely see some web gems:

Because of the high number of ground balls he has been able to induce, Thornton is uniquely positioned to take advantage of an infield defense that is arguably the best in baseball. The Cubs as a team have allowed a .224 batting average on ground balls this year, second-lowest in baseball. Their -.027 gap between that batting average and their expected batting average is also the second-lowest. 

Is Trent Thornton getting lucky? Almost certainly. With the highest ground ball rate among the pitchers on the active roster, though, he should be expected to be getting a little bit lucky, given the infield defense behind him. 

You create your own luck. Rather, Alex Bregman, Dansby Swanson, Nico Hoerner and Michael Busch create it, for Thornton. He's not a relief ace. Still, I have almost talked myself into him being a serviceable bullpen arm for the Cubs for the remainder of the season. That alone has a lot of value to the team right now.