Why Brayan Bello is one of MLB’s best relievers, but one of its worst starters

If you have consumed any local Boston Red Sox content in the last two months, you’ve probably seen, heard or biometrically absorbed a version of this stat:

Brayan Bello in 35.2 IP as a starting pitcher: 10.35 ERA, .356 opposing batting average, allowed 10 home runs. Brayan Bello in 29.2 IP as a bulk relief pitcher: 0.91 ERA, 2.04 opposing batting average, allowed one home run. 

It’s one of those stats that makes you scratch your head and then scratch it some more, one that makes you peer from over your spectacles and say “hmmmmm” loudly as you slowly tilt your head and bring a contemplative hand to your right cheek. In a similar sample size, Bello is roughly 10 times better as a bulk reliever than he is as a starter; his fWAR at each slot is inverted: -0.7 as a starter to 0.7 as a reliever. 

Brayan Bello has been dramatically better as a bulk reliever

It is also a stat that is so obviously telling that nobody has bothered to explain to me why it is happening, which is what we are going to do today. I am not content with just saying Bello has a mental block to starting games and an opener completely fixes his nerves; or, if we are going to say that, we have to at least base that claim in some evidence. Because this is far too large a change in productivity to be purely mental. There are mechanical changes afoot. 

Using openers to support a struggling starter is a relatively modern concept, and has been supported by Sabermetrics enjoyers since the early 2010s and perhaps earlier. Lots of data suggests the first inning is a danger zone, and the more times a starter sees the top of the order the more likely they are to create offense off of pitches that will get easier to hit over the course of an outing. But I’m skeptical that a more than 10x improvement is simply explainable with “oh yeah, he didn’t see Juan Soto until the fourth inning.” Bello has clearly been approaching his relief appearances differently.

It’s important to note that when I say “Bello the reliever” versus “Bello the starter,” I’m not actually talking about some sort of four-out guy in the bottom of the sixth; Bello is very much a bulk reliever, a role that has similar goals to a starter. That is, however, what makes this change so hard to explain — if he was coming in to grab two outs in a late inning, his approach would look radically different. So let’s get down to business; our first stop is pitch usage, which could cue us into what Bello is thinking with how he’s attacking hitters:

That “slider” is actually a sweeper that gets lumped in with slider numbers, but whatever you want to call it, it has been Bello’s best pitch (and only positive pitch) in 2026 by run value, while his four-seamer and cutter have been killing him all year. Bello’s cutter has among the lowest overall movement, horizontal and vertical, in the Majors, and a gander over at his pitch location chart shows how most of his cutters end up dead center. It doesn’t take a PhD in orbital launch mechanics to know why that pitch has produced an .806 slugging percentage. 

To my surprise, though, Bello’s relief appearances have actually seen a decrease in sinker usage, which was his best pitch in 2025; he actually throws it harder than his four-seamer (which has seen similarly disastrous results like the cutter in 2026), and that feels like the obvious pitch to lean into. It’s still his most used pitch, but I’m surprised that slashing the cutters in half didn’t trickle into some sinkers. Instead, he’s funneled them into the sweeper.

Bello's pitch usage only tells part of the story

Arm angle, shadow zone and spin rates are all only marginally different, and I also looked into Bello’s splits against left and right-handed hitters (he has really struggled against lefties in his career), but they were just a microcosm of the larger point. He’s been basically 10x better against each as a reliever.

So … what’s the upshot? Just more sliders, fewer cutters? Yeah? Leaning into the slider is a fine enough explanation for some of Bello’s improvement, but I couldn’t get over the fact that Bello had thrown hsi worst pitch, the four-seamer, more as a reliever and improved 10 fold. So I looked at the four-seamer specifically, fired up the pitch modeling data and man-oh-man did we find what we were looking for. Check it out:

Pitching+ is a pitch-modeling concept created by Eno Sarris and Max Bay to determine how well a pitcher is executing on each of their pitches in terms of location and stuff. A separate but related model is Location+, which shows how well a pitcher is placing each of their pitches relative to league average, which would be 100. Don’t get scared by all the jargon, because it all just (basically) means this: can he throw [insert pitch] where he wants to?

Bello has been placing his pitches much better in relief

Bello’s Location+ score on his four-seam fastball as a starter was a disastrous 66, which is 34 percent below average. His Pitching+, a separate model which factors in stuff and count, on his four-seamer is a cataclysmic 38. It’s not shocking given that run value, but it shows just how literally Bello is struggling to make the four-seamer competitive. 

As a reliever, his Pitching+ skyrockets to 86 on his four-seamer. Still below average but a titanic improvement. His Location+, though? It more than doubles from 66 to 122. Bello has completely fixed his ability to place his pitches, and the trend we just observed for his fastball exists across all his pitches to a lesser extent. Bello has figured out how to place his pitches dramatically better as a reliever... except for his curveball and (hilariously) that gosh dang cutter!

You’re probably screaming at your computer screen right now “BUT WHY?” because while we’ve pretty well established why he’s been so much better, it doesn’t magically explain why he can suddenly command his pitches. And to that I say … I don’t know! Could be something the Red Sox pitching coaches have worked on mechanically, could be him not trying to compensate for past bad performances and yes, it could also be confidence and execution of what he already knew how to do.

Look: sometimes you need 1000 words to say what everyone in Boston media was already saying: Bello the reliever has an ability to execute on his game that Bello the starter simply does not. But now you can go to your local pub, sit down with your friends for Bello’s next appearance and say that with hundreds of data points of evidence behind it. Don’t all thank me at once. 

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