ST. PETERSBURG, Fla. — The 2020 year wasn’t over yet when Vladimir Guerrero Jr. understood what he needed to do.

He was, by his own admission, overweight. And the effects on the area were obvious : The Toronto Blue Jays moved him across the diamond, from his longtime position of third base to first. He had a hard time scoring from first on a hit at the gap. He struggled to reach low pitches while hitting. He grew tired after fielding 20 ground balls in practice.

In late September, a few weeks before his team appeared in the playoffs, Guerrero sent a text message to someone he had never met, a private trainer called Junior Rodriguez, and asked to work with him at the off-season. That was the start of his trip — with aids from individuals like his grandma and Rodriguez — to bounce back from a season not befitting a player of his talent.

1 day, I woke up from bed, looked in the mirror and said, Thats it, Guerrero, 22, said in a recent video call with reporters.

Between the playoffs in October and spring training in February, Guerrero shed 42 pounds from his 6-foot-2 frame, dropping to roughly 245 lbs. He’s currently gliding around the bases, doing the splits to vacuum up low throws at first, feeling spry and acting like one of the best hitters in baseball. Entering Monday, he was hitting.338 with more walks (16) than strikeouts (13) and had reached base a M.L.B.-best 43 times.

He’s still eating his grandma’s home cooking — who could say no? — but in smaller portions and with healthier choices mixed in, and fewer are late-night foods.

He looks stronger, and hes lighter on his feet moving and running around, Guerreros grandmother Altagracia Alvino said in Spanish at a recent phone interview. She added “And what pleases me is that he’s continued with his diet. ”

Alvino has continued to reside with her grandson this season. The 10-11 Blue Jays are using their spring training facility in Dunedin, Fla.. , as their regular-season home with the United States-Canada border closed for nonessential travel due to the coronavirus pandemic. The Guerreros own a house nearby in Tampa. (And unlike last year, if the Blue Jays relocate to their Class AAA affiliate in Buffalo this summer, Alvino said she intended to accompany Guerrero.)

Since Guerrero was a youngster, Alvino said, he has had the exact same body type. With preternatural skills that belied his age, he shot through the minor leagues and was the top prospect in baseball by 19. He reached the majors with much fanfare in 2019 and dazzled fans with his prodigious power during that year’s Home Run Derby.

Over the last few decades, though, Guerrero’s weight grew. The Blue Jays had invited and worked with him to lose weight — and he failed in spurts. However, in baseball’s four-month pause this past year due to the pandemic, he grew heavier — for which he has stated he apologized to his teammates.

For the last couple decades, they knew I was coming here from shape, Guerrero told reporters in February. I didnt prepare me very well. ”

From the abbreviated 60-game 2020 season, the Blue Jays moved Guerrero into the less demanding position of first base, and often out of the infield entirely, at designated hitter. He posted decent numbers — a.262 batting average, nine home runs, a.791 on-base plus slugging percentage — and helped guide the Blue Jays to their first playoff appearance in four years. But he and his team knew he was capable of so much more.

Deep down inside, he knew he had a bad year, Rodriguez said in Spanish in a phone interview. “Not a very bad year, but in a physical level of performance, he was tired of being suppressed and feeling bad. ”

Initially, Alvino wasn’t quite sure why Guerrero’s weight increased last year. Eating her cooking — she removes excess fat from any meat she cooks, and sparingly uses oil — Alvino said Guerrero lost some weight. However he then hit a plateau. She said the Blue Jays even asked her if anything had changed, and she insisted nothing had.

Later, however, she said, she poked around and found two possible culprits: light beers in the refrigerator near the pool table at home, where Guerrero hung out with family and friends, and late-night snacking after she had fed him dinner.

I told him, My love, what Im doing with my hands, youre destroying with your feet, Alvino said, repeating a common saying in Spanish.

In September, Guerrero connected with Rodriguez, a personal trainer who has worked together with other big leaguers such as Ketel Marte, Ramon Laureano and Santiago Espinal, one of Guerrero’s teammates.

Soon after Guerrero returned to the Dominican Republic following the Blue Jays’ playoff exit, Rodriguez swung from the Guerreros’ house to meet his new client for the first time. Rodriguez felt what would make the biggest difference was getting Guerrero to purchase into lifestyle changes, so he was blunt.

I reduced his sanity a little, but I was being sincere with him, Rodriguez said. “I told him: ‘ you ought to be ashamed that lovers are underestimating you and talking bad about you, and you are among the best prospects in the big leagues. Let’s devote for this; I’ll devote to you and you to me. I don’t want this just for 2021, but eternally. You’re going to sleep to be an athlete, you’re going to eat like an athlete, you’re going to train like an athlete. ’”

And to Guerrero’s credit, he did. For 90 minutes a session, four times a week in the winter, at the gym in Guerrero’s home, Rodriguez trained Guerrero. He guided him through phases of conditioning, stretching, weight lifting and jumping exercises, focusing on improving any physical limitations. For example: Rodriguez reported the lack of range of motion in Guerrero’s chest dampened his power.

Its been a change of 180 degrees, Rodriguez said. “This is a very different Vladimir. ”

Away from the gym, Guerrero became more disciplined — and the people around him helped. He increased his water intake. He stopped snacking after dinner. No more alcohol, fried food or sugar. He slashed his portion sizes, something Alvino said she had to become accustomed to. She said a longtime family friend who also cooks has also kept an eye on the portions.

Alvino said she made Guerrero cucumber-green apple-celery juice before the morning work outs and a papaya-oatmeal-cinnamon shake in the evenings. Lunch was the largest meal of the day. Although Guerrero never ate a great deal of rice, Alvino said, she started using a smaller spoon to serve him. He still got a normal amount of her stewed beans, his favorite, and fish or meat.

“If he finishes quickly, I say, ‘Are you hungry? ’” she said. “He states, ‘No, abuela, I’m OK. ’”

Guerrero still takes Alvino’s traditional Dominican meals to the stadium to share. If he’s hungry after night games, she makes him a little meal of grilled chicken or fish and vegetables. (And if she makes mangú, a dish of mashed boiled plantains, she said, she utilizes milk and water but not butter. )

Guerrero said in a recent video call with reporters that his body felt “really different” today and that he was proud of his off-season work. He said he intended to stick with the current regimen for the remainder of his career. At the plate, he said, he felt better than at the height of his minor league career (he hit.381 in a season ). And even though he’d originally hoped to return to third base, he said he was happy playing wherever the Blue Jays needed him.

I can do a lot of things I cant do before and, thank God, I’m very comfortable at the weight Im at, he said.

Blue Jays Manager Charlie Montoyo stated a fitter Guerrero had shown improvement in every aspect of his play on the field. Guerrero is hitting the ball much harder than usual this season, trailing just a small number of celebrities like Ronald Acuña Jr. of Atlanta and Giancarlo Stanton of the Yankees in that class.

Hes back to being Vladdy, grinning in the dugout, having fun, said shortstop Bo Bichette, who has played alongside Guerrero since they were teens and believed his friend had the ability to be the best hitter in baseball. I havent seen him get out and smile about his outs since Double-A, and hes been doing that much since he understands hims going to get them at a certain stage. ”

The baseball season, though, is arduously long and tests everybody. Rodriguez said he sends Guerrero reminders to remain on top of his regular. Montoyo wasn’t planning on doing the same.

Hes on a mission, and he doesnt need any help at the moment, he said of Guerrero. “He knows what he wants to do and he’s doing it. ”