Andrew Benintendi spent his first offseason as a major leaguer chained to the gym.

OK, so that's a bit of an exaggeration. The Boston Red Sox rookie left fielder -- and No. 1 prospect in baseball according to ESPN's Keith Law -- made a few trips to his hometown in suburban Cincinnati and visited some college buddies at the University of Arkansas, as most ordinary 22-year-olds would when they got time off from work.

But Benintendi also relocated to St. Louis and, at the recommendation of his agent, went through four months of intense workouts at Premier Pitching and Performance (P3, for short), an elite baseball training facility. How intense? Check out Benintendi's Twitter feed, specifically a Jan. 17 video in which he's doing curls with ropes and chains.

The crude-looking exercise, conceived by performance coach Steffen Simmons to supplement Benintendi's lifting program, was part of a larger plan. In anticipation of the rigors of a full big league season, Benintendi and the Red Sox agreed it would be best if he gained weight and added muscle. Two weeks ago, Benintendi reported he was 185 pounds, which is 15 to 20 pounds heavier than the end of last season and "the most I've ever weighed."

"It's a long season. Obviously staying healthy is the main thing," Benintendi said. "By adding some weight and staying mobile and flexible, overall that'll help in the long run."

Hey, whatever works. Because although Benintendi captivated Red Sox fans with his dark, flowing hair and a smooth left-handed swing that yielded a .295 average, .359 on-base percentage, 11 doubles, two home runs and an .835 OPS in 105 at-bats after getting called up from Double-A last August, plus a homer in his first postseason at-bat, history shows the second time around the league is typically more, well, hairy.

Law's 2017 Prospect Ranks Keith Law ranks the top 100 prospects in baseball, the best farm systems, and much more. Top prospects of 2017

To wit: After bursting onto the scene in the 2013 playoffs, Red Sox shortstop Xander Bogaerts batted .203 with a .556 OPS over the last four months of the 2014 season. Center fielder Jackie Bradley Jr. hit .198 with a .531 OPS in his first full season. Even superstar right fielder Mookie Betts dealt with early career challenges, batting .234 with a .658 OPS through 60 games in 2015, his first full year in the majors.

So, if moving to a new city on the advice of agent Jason Wood to train at P3 will help flatten the learning curve, Benintendi was more than willing.

"I felt like Andrew needed a throwing program, he needed a hitting program, and he needed a weight-lifting program that was geared toward getting him through a 162-game season," Wood said. "He's eating unbelievably well, he's getting great sleep, he's working hard. His preparation for 2017 is not even in the same ballpark as it was for 2016. I don't know what that's going to translate into -- I hope it translates into Rookie of the Year -- but his preparation is off the charts."

That's exactly what the Red Sox want to hear about their 2015 first-round draft pick. They are counting on Benintendi to bat second and, in time, join Bogaerts, Bradley and Betts as a homegrown All-Star. Oh, and if he can help replace David Ortiz's production from the left side of the plate, even better.

No pressure, kid.

Benintendi was not surprised with the Red Sox told him he needed to get stronger this offseason. Ted S. Warren/AP Photo

Benintendi is listed at 5-foot-10, a designation that even his father calls "a little gracious." No matter how much time he spends in the weight room, his body will always look more like Dustin Pedroia's than Big Papi's.

Gaining weight can be a struggle for Benintendi, so P3 co-founder Josh Kesel said he received a nutritional plan that emphasized eating more often throughout the day over more food per meal.

"With a guy like Andrew, we're really trying to balance that speed and bulk," Simmons said. "He's worked his butt off. He's worked extremely hard and eaten well. That's where that weight and that bulk is coming from."

Said Benintendi: "Honestly, last offseason compared to this offseason, I wasn't as prepared as I am right now. Going into this offseason, I had a lot of work to do."

While Benintendi's work at P3 is designed to keep his body from wearing down during a six-month season, it won't help him adjust to the new ways in which he's bound to be pitched.

After getting called up last season, Benintendi played 34 games against nine teams. Only the New York Yankees, Baltimore Orioles and Tampa Bay Rays got a look at him in more than one series. But despite the limited exposure, Benintendi said he noticed in September that he was already being attacked differently.

"I saw, from what I remember, a lot more off-speed pitches than what I expected," he said. "I'm going to take what I saw last year and hopefully learn some things and go from there."

Benintendi has a level swing that allows him to line singles and doubles the other way to left field. Like Betts, he has lightning-quick hands that help him generate rare power for a player his size. And he has a knack for making consistent contact, racking up fewer strikeouts (63) than walks (74) in 570 minor league at-bats.

"He can just adapt to any kind of pitching," said Dave Evans, Benintendi's coach for three years with a Cincinnati-area summer league team that has produced dozens of big leaguers, including Barry Larkin and Ken Griffey Jr. "In high school, he was probably seeing guys throw 75 [mph]. He comes to us, we're seeing guys throw 88, 90, 92. It doesn't make any difference to him. He's hitting line drives, putting the bat on the ball. He's just an amazing kid."

Said Red Sox manager John Farrell: "He's got some things that excite you in terms of his calmness and the ease in which he seems to play the game. I think that plays out over a long, full season."

A season that Benintendi is convinced he has prepared himself to withstand.