Red Sox pitcher Zack Kelly celebrates after escaping a jam against the New York Yankees on June 16. Kelly has a 1.78 ERA in 35 1/3 innings and has earned a role as a late-inning reliever. Steven Senne/Associated Press

BOSTON — Manager Alex Cora described Zack Kelly as “one of our best” after the Red Sox righty recorded the final six outs last Saturday in a win over Kansas City.

That high praise came just 49 days after Boston demoted Kelly to Triple-A Worcester for nine days.

Since Kelly returned to Boston on June 5, the 29-year-old reliever has a 1.45 ERA (18⅔ innings, three earned runs). He has held opponents to a .139 batting average, a .188 on-base percentage, a .246 slugging percentage and a .435 OPS.

He pitched well even before his demotion. The Red Sox sent him to Worcester on May 26 despite a 2.16 ERA (16⅔ innings, four earned runs) and a .180 batting average against in 11 outings.

So why’d the Red Sox demote him in the first place?

It’s sometimes a numbers game (not a statistics) when a reliever has minor league options remaining. Kelly pitched three innings May 25 after a short outing from starter Nick Pivetta. Long reliever Chase Anderson hurled four innings the day before after a short outing from starter Kutter Crawford.

Anderson (no options) and Kelly (options) weren’t going to be able to pitch for a few days after their multi-inning outings. The Red Sox needed a fresh arm, so they signed veteran Brad Keller as a long reliever and sent Kelly to Worcester.

Is it really even a demotion if a player has options and it’s not based on performance?

“It was definitely a demotion,” Kelly said. “Whether performance-wise or not, you’re still in Triple-A, not in the big leagues. You’re not getting service time. But yeah, it’s frustrating no matter what the case may be. Like last time, I think Chase went (four innings) on Friday, I went three on Saturday, and then Sunday, I was in Worcester. That’s just the reality of the game. Everybody in here has had to deal with being in that position.”

Despite feeling frustration, Kelly understood where Cora, chief baseball officer Craig Breslow and pitching coach Andrew Bailey were coming from when they called him into the office.

“I couldn’t say anything because they were right,” Kelly said. “They needed a fresh arm and I know that’s how the game goes. As frustrated as I was, they were right. They needed a fresh arm and they got it. But it is what it is.”

The second half of the season begins Friday in Los Angeles when the Red Sox play a three-game series against the Dodgers. With both Justin Slaten and Chris Martin sidelined because of right elbow inflammation, Kelly has the chance to step into a setup role. Kelly already has appeared in the seventh inning or later in all four of his outings in July.

The righty began the regular season at Worcester before Boston recalled him for the first time on April 24.

Even though he pitched well during that first stint in Boston, he knew he was as risk of being demoted at any time.

“You know that it’s always a possibility,” Kelly said. “It’s hard not to think about it, but at the same time, you’ve just gotta kind of force yourself to not think about it. Because when you start thinking about it, that’s when your performance goes down. When I came back up the first time, I was a little worried about it. Obviously that happened. But when I came back, the mindset is just to try to pitch to force myself to stay here. I feel like as long as I can do that – and just continue to pitch and not think about that and just continue to pitch as well as I can every time I go out – then I can live with whatever happens.

“I don’t want to sit there and get sent down and then know in the back of my head that I was thinking about it and probably didn’t pitch the best that I could,” Kelly added.

He said having options also can work in a player’s favor. A player without options must be designated for assignment if he does not remain on the active 26-man roster. If he goes unclaimed on waivers, he would be outrighted off the 40-man roster, making it less likely to be promoted again.

“Options are a good thing and a bad thing,” Kelly said. “Later in your career, the longevity, it’s better to have them. Obviously early in your career, you don’t want to have them. That’s just the reality of the game. It’s the business side of the game and you understand that.”

Kelly’s only problem this season has been his control. He opened the season with just one hit allowed in nine scoreless innings for the WooSox, but walked eight batters.

He then walked 10 batters in 16⅔ innings during his first stint with Boston.

“I didn’t really have much of a spring training,” Kelly said. “I didn’t have a full spring training. I got hurt March 2, and then I threw one inning in a backfield game before Opening Day in Lehigh Valley.”

Kelly dealt with an oblique injury in March.

“So it was like three, three and a half weeks, I didn’t compete in games,” Kelly said. “And then the zone at Triple-A is small. The ABS (automated ball-strike system) is small. There’s no forgiveness there. So I think that almost forced me to try too hard. I walked some guys when I got up here to the big leagues. But I think those were more competitive than the ones I had in Triple-A. In Triple-A, I had a lot of four- or five-pitch walks. I don’t really think I had a ton of those here. But since I came back up, I’m kind of working on some delivery stuff. Pretty consistently, I feel much better living in the strike zone.”

Indeed, Kelly has just four walks in 18⅔ since his second promotion, and only one walk in his last 10 outings (14 1/3 innings).

Kelly has held opponents to a .118 batting average (4 for 34) against his sweeper, a new pitch.

“My arm slot, I felt like the sweeper would work well for me,” Kelly said. “But it’s just getting the consistency down. Getting the confidence not only to throw it in a game, but being able to throw it in those 2-1 and 3-1 counts. That’s the biggest thing. Because if you have a pitch you only really throw in two-strike counts, I don’t think you’re that confident in it. I think when you’re confident in something, you feel like you can throw in 3-1, 3-0 even. … So I think that was the biggest thing for me. It took a little bit, but I feel really good where it’s at now.”

Opponents are 8 for 40 (.200) against Kelly’s four-seam fastball, which has averaged 96.2 mph, per Baseball Savant. They have gone 3 for 26 (.115) against his changeup and 5 for 26 (.192) against his cutter.

Kelly has been attacking the strike zone with all his pitches.

“I think it kind of comes with being more confident in my stuff now than I really ever have been,” Kelly said. “I’ve always been confident in my stuff, but there were definitely times where in the past, even a few years ago, I wouldn’t necessarily be hesitant to throw the ball in the zone, but I would definitely try to nibble a little bit more than I probably needed to. But right now with how I feel about my stuff, I feel confident going right at guys. I try to live in the zone as much as I can. And it’s working out so far.”