Mickey Gasper, center, gets ready to field a ground ball as his teammates look on during the first workout of the season of the Portland Sea Dogs on April 3. Gasper made his MLB debut on Monday at age 28. Shawn Patrick Ouellette/Staff Photographer

BOSTON — Red Sox rookie Mickey Gasper fell behind 1-2 in his first major league plate appearance Monday while pinch hitting in the 10th inning.

With runners at the corners and one out in a tie game, Gasper laid off a slider in the dirt from Rangers left-handed reliever Walter Pennington to even the count 2-2.

“He took a really, really good slider,” Rob Refsnyder said. “Most guys, in that moment, debut – I know I would have swung over the top of that one.”

Mickey Gasper, the Yankees’ 2018 27th round draft pick, spent six seasons in the minor leagues before his MLB debut on Monday. Jeffrey McWhorter/Associated Press

Gasper drew a seven-pitch walk to load the bases. The Red Sox earned a 5-4 walk-off win two batters later when Refsnyder smashed a two-out RBI single into the left-center field gap.

“That at-bat by Mickey, in that moment to be able to walk, that was impressive,” Red Sox Manager Alex Cora said. “That’s the reason he is here.”

Refnsdyer added, “That was a huge at-bat. He did his job. We gave him a beer shower to celebrate his debut. Just a fantastic job him by honestly.”

Boston made nine roster moves before Monday’s game, including promoting Gasper from Triple-A Worcester for his first MLB stint. The Yankees’ 27th round draft pick in 2018 out of Bryant University spent six seasons grinding in the minor leagues before finally receiving the call to the big leagues at 28 years old.

The actual call came from WooSox Manager Chad Tracy about five hours before first pitch.

Mondays are scheduled off days across Minor League Baseball. Gasper was at his apartment. He had just finished a couple of off-day chores.

“It had to have been around 2 o’clock-ish, 2:30. Yeah, it was late,” Gasper said. “I had gotten my hair cut, went grocery shopping. … I was just getting relaxed. I was going to cook in a little bit. I was just getting comfy and Chad Tracy called me up and let me know.”

Gasper put on a sports jacket and a collared shirt, then headed to Fenway Park.

He said his dad told him when he was younger to “act like he’s been there before.”

“So I figured I had to dress up,” Gasper said. “This is a big day.”

Cora added, “You should have seen him when he walked in. I’ve never seen a call-up as dressed up as he was. It was sports coat and all that. I’m like, ‘Where were you?’ He was like, ‘At the apartment.’ I was like, ‘Wow, good for you.’ But all kidding aside, the at-bats are real.”

Gasper’s excellent at-bats at Worcester began catching the attention of the front office and Cora about a month and a half ago.

The catcher/first baseman – who also recently has played some second base – had ridiculous Triple-A stats. He batted .401 with a .515 on-base percentage, .664 slugging percentage, 1.179 OPS, eight homers, 12 doubles, 32 RBI and 36 runs in 40 games (171 plate appearances) for the WooSox. He had more walks (29) than strikeouts (16).

He began this season at Double-A Portland after Boston selected him from the Yankees in the minor league phase of the Rule 5 Draft in December. He had 55 walks and 38 strikeouts in 84 games (347 plate appearances) between Portland and Worcester.

Refsnyder pointed out that Gasper “seemed calm” during his 10th inning plate appearance.

“I was so calm up there,” Gasper said. “I don’t know what was going through my head. I just felt good. I guess I felt like I was at home. I don’t know. I was relaxed, saw the first couple pitches.”

The Rangers had a lefty and righty warming in their bullpen in the bottom of the eighth.

Cora initially thought Rangers Manager Bruce Bochy would bring in the lefty to face David Hamilton. Cora told Gasper he was going to pinch hit. But Bochy instead put in righty Kirby Yates and Hamilton stayed in the game to bat.

“At least he got a false start so he could calibrate his heart again and be ready,” Cora said.

Gasper added, “I’d say my heart jumped a little bit there. But once I got sat back down, I calmed down, took a few swings and I kind of had the feeling I was going in. So I was mentally prepared for that lefty.”

Gasper always believed this day would come.

“I’ve always thought the big leagues were realistic,” he said. “People have told me that I’m a big league hitter before. I’ve always been oblivious to doing anything other than playing baseball. So anything short of the big leagues just didn’t seem like a possibility for me. I’ve always had the confidence and belief in myself that this is what I want to do. And just had to set my mind to it and really push myself as much as I possibly could.”

Gasper graduated from Merrimack High in New Hampshire and Bryant University. He actually grew up a Yankees fan in New Hampshire because his parents are from New Jersey.

His parents no longer live in New Hampshire. They moved back to New Jersey in 2019. And so they were unable to make it from there on such short notice to attend Gasper’s first MLB game. They planned to attend Tuesday’s game.

Justin Snyder, Gasper’s best friend from college, was able to attend the game.

“He lives up in Malden. He said, ‘Leave me a ticket. I’ll be there.’ So I got to see him and it’s pretty special sharing a moment with him because he’s just one of my biggest supporters,” Gasper said.

Gasper received the lineup card. He also plans to keep his cap as a memento.

“It was fun. Never done anything like that,” Gasper said about the beer shower. “That was fun.”