Why Red Sox rookie Brayan Bello could end up in the 2023 rotation

In five starts since his Aug. 24 return from the injured list, Brayan Bello has a 2.84 ERA and has struck out more than a quarter of the hitters he's faced.Barry Chin/Globe Staff

As Eric Hosmer watched a Mariners game while eating in the Red Sox clubhouse last week, he did a double-take while watching Seattle star Luis Castillo carve up the Padres over six shutout innings. In Castillo’s repertoire — a high-90s sinker that sawed off righties, along with a nasty changeup and slider — Hosmer was struck by the similarities to Red Sox teammate Brayan Bello.

That’s high praise for a pitcher who is 1-6 with a 5.10 ERA entering Tuesday’s start against the Reds. Castillo, after all, is a two-time All-Star with a 2.68 ERA this year. Yet what Bello has shown in his brief time in the big leagues has made a believer of veteran teammates.

“You see guys come up and you’re like, ‘Man, this guy’s got good stuff. If he can learn how to pitch, he can be really good,’ ” said Hosmer. “But you can tell [Bello] already knows what he wants to do with his plan. You can tell it’s all about him executing his pitches, and he’ll be really effective. And I think he’s going to continue to get better and better.”

The optimism surrounding the 23-year-old Bello is widely shared throughout the organization, a glimmer of hope in an uncomfortable stagger toward the 2022 finish line. Bello’s repertoire dazzles. He features a high-90s sinker that harbors similarities to Castillo and Yankees closer Clay Holmes; a nasty, swing-and-miss changeup; and a slider that flashes potential as a solid third pitch. Yet members of the Red Sox are encouraged not just by Bello’s stuff, but by his intellect.

Bello, after all, is ahead of schedule. The Sox thought a late-season call from Triple A Worcester might be a possibility, but with Nate Eovaldi, Rich Hill, Michael Wacha, Garrett Whitlock, and Chris Sale all on the injured list in early July, that timetable was accelerated.

“It was by necessity,” said pitching coach Dave Bush.

Bello, walking in for a July start next to former Sox backstop Christian Vazquez, has had plenty of reasons to smile lately.Jim Davis/Globe Staff

Bello, after just 11 Triple A starts, got thrown into the deep end and initially struggled to tread water. He allowed 16 runs in 16⅓ innings across his first four outings, a struggle that reflected some elements beyond his control (bad luck on ground balls) and some that weren’t (a 13.3 percent walk rate).

He responded not by retreating within himself, but instead by engaging teammates and staff members to identify ways to improve.

“Of course I have to make adjustments,” Bello said through translator Carlos Villoria Benítez. “You can’t imagine how many things I’ve learned here.”

As part of that process, Bello expressed particular appreciation for Eovaldi, who reached the big leagues at age 21 and has been eager to share insight with the young pitcher.

“I definitely look up to [Eovaldi]. I really admire him and I try to copy every good thing that I can. I try to watch every outing, every bullpen, and try to copy the good habits that he has,” said Bello. “And of course the way that he attacks batters is something that I really like, too.”

Eovaldi reciprocated the pupil’s enthusiasm.

“Once I started talking to him, it’s like he’s right over your shoulder, always watching and trying to learn to be better,” said Eovaldi. “You can tell he’s got that hunger, and he really wants to learn. We all see the competitiveness in him and that fiery energy he brings out there. You just want to try to make sure that he can learn as much as he can from an early stage because it took me five years [to learn how to be a big league starter]. If it only takes him one, that’s a huge advantage for him.”

Examples of the lessons being absorbed are numerous. Eovaldi encouraged Bello to be more aggressive attacking the strike zone; he’s gone from throwing 59.0 percent strikes in July to 63.1 percent in August and September.

As part of that effort, Bush suggested Bello might benefit from shifting from the third- to the first-base side of the rubber — a common position for pitchers with his sinker movement. That would give pitches with tremendous arm-side run a better chance of either landing in the strike zone or looking like strikes.

Bello tried pitching from there in his next bullpen session, then he took the change into the game. Since he returned from the injured list in late August, Bello’s release point has been roughly 6 inches toward the first base side compared with where it was in July, with excellent results.

In July, Bello’s slider was promising, but inconsistent. He proved open to altering his grip and wrist position in an effort to improve it. With most of his pitches having east-to-west movement, Bush and Bello discussed the potential value of adding a curveball for a north-to-south offering. Bello spun a few in the bullpen, continued his conversations about curveball grips with teammates such as Hill in the dugout in subsequent days and threw two in his last start.

Pure. pic.twitter.com/pEK1cBmFTx

— Michael Hurley (@michaelFhurley) September 13, 2022

In the same start, Bello altered his delivery to employ a quick pitch. When he did so, Eovaldi and Bush turned to each other, curious if Bello had discussed employing the tactic with the other.

Neither had. The rookie had deduced the value of disrupting hitters’ timing with a change in the pace of his delivery by watching Eovaldi, Hill, Nestor Cortes of the Yankees, and others.

For Bello, ideas are quickly translated into action.

“With all these things, none of them are a big piece in their own right, but you add them all together and they make a more complete guy,” said Bush. “I think that’s the stuff that shows in his adjustment and development to the big leagues.”

The product of those adjustments has been impressive. In five starts since his Aug. 24 return, Bello has a 2.84 ERA with a 25.2 percent strikeout rate and 9.3 percent walk rate. He still has yet to allow a homer, the first Red Sox rookie since 1978 to make eight starts to begin his career without giving one up, and his 57.8 percent groundball rate is the fourth highest among pitchers who have made at least five starts this year.

The performance may be altering his timetable for a more permanent spot in the Red Sox rotation. The way in which Bello used his initial struggles as a platform for improvement — along with an arsenal that is clearly that of a big league starter —make an increasingly compelling case that he can open next season in Boston.

“There’s definitely a path where he’s going to be a good big league starter for us for a long time,” said Bush. “What that means for next year I don’t know, because I don’t know who else we’re going to have. I don’t know who’s coming back, who we’re going to sign, but he’s certainly put himself in a spot to compete for a rotation position next year.”

For Bello, that potential opportunity represents a significant achievement. Not just for what it says about his present abilities, but for what it would mean to his continued pursuit of improvement.

“I think I’m a big league pitcher,” said Bello, “[but] I’m still growing up. I’m still improving every day.”