PHOENIX -- The development was noted in the Minor League section of the Brewers media guide under Antoine Kelly's name.

Underwent pectoralis minor release surgery and a first rib resection surgery on Nov. 5, performed by Dr. Gregory Pearl.

Translation: Kelly, the 21-year-old power left-hander who recently ranked No. 6 on MLB Pipeline’s list of the Brewers' top prospects, underwent offseason surgery for thoracic outlet syndrome.

“We think the prognosis is good,” Brewers president of baseball operations David Stearns said. “Thoracic outlet surgery has become a more routine procedure over the past couple of years. The timeframe is still a little bit unknown and it can vary pretty widely from player to player, but we do think the prognosis is good."

As for a timeline, Stearns said, “I think it’s unlikely for him to be ready by the start of the Minor League season. That is fair. I think we do envision him pitching over the summer. Exactly when, we don’t know.”

Kelly has been rehabbing at American Family Fields of Phoenix amid the hustle and bustle of Spring Training. 

"It has been difficult to not be able to take part," he said.  

“I tried to keep [the surgery] low key because I thought I would be ready right now, but I guess they’re just taking it really slowly,” Kelly said. “I mean, I would rather keep it slow than hurry up and have something else happen. I’m pretty good now. When I had the surgery, I was a little bummed out. There’s nothing I can do about it now but wait to get better, take it slow."

The thoracic outlet lies at the lower part of the neck, beginning just above and behind the collarbone and extending into the upper arm and chest, and thoracic outlet syndrome results when the nerves and blood vessels in this area are compressed. The result is pain, weakness, fatigue and numbness or tingling in the arm or hand, particularly with activities in which the arm is elevated.

Kelly’s symptoms date back to his participation in last year’s alternate training site, when he noticed that his left arm was swollen. But he felt fine and was pitching great, so he kept going. He didn’t stop until after his first outing in the fall instructional league. 

“It felt weird,” Kelly said. “I threw that one time in instructs and the next morning, I woke up and was brushing my teeth and my arm was huge and my hand was purple. I was like, ‘What is this about?’ I got scared, so I had to tell them.”

Kelly first complained his arm “felt a little weird” during the fall instructional league, Stearns said, and while he didn’t necessarily experience a drop in stuff, testing resulted in a diagnosis of TOS.

During surgery to correct the condition, which remains relatively rare in baseball compared to elbow or shoulder surgeries, both the uppermost of the ribs and some adjacent musculature are removed to clear space for the nerves in the thoracic outlet.

Among the Major League pitchers who have undergone the procedure are Chris Carpenter, Mike Foltynewicz, Jaime Garcia, Matt Harvey, Phil Hughes and Chris Young.

Kelly was 20 at the time of his November surgery. He turned 21 on Dec. 5.

He was a 6-foot-6 project with 19.1 strikeouts per nine innings for Wabash Valley (Ill.) Community College when the Brewers drafted Kelly in the second round in 2019. He pitched that year almost exclusively with a fastball but spent 2020 polishing his game at the Brewers’ alternate training site, honing a changeup and working to hold runners and field his position. Kelly so impressed that some club officials considered him a contender with outfielder Tyrone Taylor for the unofficial title of alternate site MVP.

“He really was one of the standouts of the alternate site as a really young player there,” Stearns said. “Whenever we would call someone up from the alternate site last year, inevitably the player, whether it was a pitcher or a position player, would comment on Antoine Kelly and the type of stuff he was throwing.

“Any injury is disappointing for a player and I think it’s very disappointing for Antoine because he had worked so hard and was geared up for a really good and complete 2021 season. Unfortunately, that is going to be delayed and truncated a little bit, but our goal is to get him back healthy, get him going, and hopefully make the most of the time he does have in ’21."

Kelly isn’t sure when he’ll be back on a mound. He only hopes it is “really soon.” 

“I hope I’m not missing too much,” Kelly said. “The way I am, I hate being sidelined and watching everyone throw. The big-league Spring Training? It’s killing me right now. The early [Minor League] campers? Everybody is throwing and I can’t do any of it. We just have to wait and see.” 

He has a reminder of his surgery tucked in a drawer at his mom’s house. Like the other pitchers who underwent thoracic outlet surgery, doctors sent him home with a piece of his rib. 

“It’s still in the little bottle they gave it to me in,” he said. “I’m too scared to clean it and stuff. It’s a little memory, I guess.”