ST. PETERSBURG, Fla. — Darwinzon Hernandez was beaming Thursday afternoon. After a three-month stretch that included a somewhat surprising demotion, a lot of work behind the scenes at Triple-A and knee surgery, the lefty reliever was back in the majors with the Boston Red Sox for the first time this season.

Hernandez was activated Thursday afternoon to take the roster spot of Josh Winckowski, who tested positive for COVID-19. Hernandez pitched a 1-2-3 eighth inning in Boston’s 5-4 loss to the Rays, striking out two batters while throwing just 12 pitches.

Hernandez’s first big league outing of the year was a good sign, as he pounded the strike zone in a way he seldom did previously. When the Red Sox sent the 25-year-old to the minors to start the season, they did so in an effort to get him to refine his delivery and work on his command. Manager Alex Cora said at the end of spring training that Hernandez needed to throw fastballs up in the zone and breaking balls down. His walk rate (7.4 per 9 innings) in his first three big league seasons was an issue. So the Red Sox decided he needed to spend a significant amount of time improving away from the big league club.

Hernandez started games in short stints for Triple-A Worcester and was starting to turn a corner before injuring his knee in a bullpen session in mid-May. He underwent surgery to repair a torn meniscus on May 19 and didn’t return until July 4.

The reports from Worcester were encouraging in the last couple weeks, as have the results. Hernandez didn’t allow a run and struck out six over his last four appearances in Triple-A (four innings).

“He has been throwing the ball well,” Cora said Thursday. “I actually talked to him. (WooSox pitching coach Paul Abbott) is very pleased with the progress. His delivery has been clean for a while. Obviously, he had surgery and all that stuff, but they feel like he’s very direct to the plate and he has been consistent with it. The stuff is always there. It’s a matter of him being consistent in the strike zone. So far, so good.”

For Hernandez, who was a vital part of Boston’s bullpen in 2019 and entered the last two seasons with high expectations, heading back to the minors was a humbling experience. At the end of spring training, Cora said the organization still believes Hernandez is a prospect who needs more development even though he has so much big league time. Hernandez had to embrace that in Triple-A.

“Of course it’s really hard, but when you know what you need to work on and you work every day to get that fixed and get back to the big leagues, the time goes by very fast,” Hernandez said through interpreter Carlos Villoria Benítez.

Working with Abbott, Hernandez tried to develop a more consistent delivery to get the most of his above-average arsenal, which features a four-seamer and wipeout slider. The Red Sox believe mixing Hernandez’s stuff with improved control could be a deadly combination. And with a knee that the lefty claimed as “good as new,” Hernandez thinks he’s in a much better place than when the Red Sox last saw him.

“Before, I was too stand-up. Now, I’m a little bit closed,” Hernandez said about his delivery. “Before that, I was trying to overthrow the ball. Right now, I’m more calm and trying to just throw without any exaggeration or anything like that.”

It won’t be easy to carve out a major role in the Red Sox bullpen, which already includes three lefties (Austin Davis, Jake Diekman and Matt Strahm) and soon may add another when Josh Taylor comes off the injured list. But he has the ability to throw multiple innings and has a track record of major league success. The role will work itself out. For now, Hernandez is just happy to be back in the majors.

“It feels great,” he said. “That’s what you want when you’re down there. You’re trying to get back to the team and work as hard as possible to get to the big league team. I’m really happy to be here right now.”

THE RED SOX welcomed two of their most important pitchers back to the lineup.

Boston activated right-handers Nathan Eovaldi and Garrett Whitlock off the injured list. To make room on the roster, the Sox placed lefty Matt Strahm (left wrist contusion) on the 15-day injured list, retroactive to July 13, and optioned righty Phillips Valdez to Triple-A Worcester.

Eovaldi will start Friday night’s series opener against the Yankees. It will be his first start since June 8; he was placed on the IL because of lower back and hip issues four days later. Whitlock, whose last nine appearances came as starts before he injured his hip in mid-June, will be in the bullpen as a multi-inning weapon.

Strahm was injured Tuesday night when he was hit in the wrist with a 98 mph Taylor Walls line drive. X-rays were negative, and the Red Sox were hoping to avoid putting him on the injured list, but he was unable to play catch over the last three days. He won’t be eligible to return until July 28.

Boston now has 10 players on the injured list, including eight pitchers (Strahm joins James Paxton, Josh Taylor, Matt Barnes, Rich Hill, Tyler Danish, Michael Wacha and Connor Seabold).

Valdez has bounced back and forth between Triple-A and the majors this year and owns a 4.41 ERA in 16⅓ big league innings. This is the fifth and final time he can be optioned without being exposed to waivers, so the Red Sox risk losing him the next time they call him up and send him down.