“I want people to be like, ‘God d---! What was Garnett doing in the offseason?’ ” Joshua Garnett said this week.

The 49ers guard and former first-round pick out of Stanford was shut down for the season following an August knee injury. But that doesn’t mean he hasn’t been working.

Garnett’s knee issue wasn’t overly serious – “Just a clean-up,” he said – and the 49ers could have brought him back to the active roster at some point in 2017. Instead, the team decided on a long-term, holistic approach, one that not only gets Garnett’s knee feeling good again but also reshapes his body so he’s a better fit for the 49ers’ mobile, zone-blocking scheme.

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Despite not being able to run in the weeks after surgery, Garnett already has dropped 20 pounds and dramatically reduced his body-fat percentage to the point where even his face looks slimmer.

“It’s just been unbelievable to me the change in my body composition, how I see football, my strength,” he said. “I mean, I feel stronger than I’ve ever been, more lean than I’ve ever been. It’s exciting to me because it’s not even December yet and I already feel like I’m in the best football shape I’ve been in in years. So I’m like, ‘Jeeze, if I can continue to do this, then I’m going to be a monster by the time OTAs come.’ ”

The 49ers have sent more than 20 players to injured reserve since training camp, more than any team in the league.

Garnett represents a silver lining to that list in that very few of the injuries threaten to bleed into 2018. Most – including key players like linebacker Malcolm Smith and defensive end Arik Armstead – will be ready for the start of the offseason program in April, which gives the 49ers what is tantamount to a second wave of free agency.

Garnett was competing for a starting guard spot when he was hurt. Heading into the 2018 season, he figures to compete with holdovers Laken Tomlinson and Erik Magnuson, who was placed on injured reserve this week, and anyone else the team acquires in the offseason. Starting right guard Brandon Fusco is scheduled to be a free agent.

Garnett said one of the major pieces of his rebuild involves nutrition.

While dinnertime previously left him wondering what to eat – and making bad decisions because of that – the 49ers’ team nutritionist has set up a program where all of his meals are delivered to his house at the beginning of the week.

Now his dinners are settled – salmon on Monday, chicken breast on Tuesday, etc. – eliminating a need for an impromptu visit to the local fast-food joint.

“It’s gotten to the point where you’re eating so well that if you go to Taco Bell or McDonald’s or something your stomach is just destroyed,” he said. “… You don’t want that anymore.”

Garnett noted that Stanford’s power-based offense called for 325-pound mauling guards. The 49ers’ system is different, which means that he must be different, too.

“In this offense, you’ve got to be fast and athletic, you’ve got to be able to bend and move,” he said. “Obviously, the less body fat you have the more mobile you can be, the more you can run, be fast and be in better condition. I definitely feel the more I’m able to change my body composition, the more explosive I’m able to become. The more explosive I am, the better I can perform in this offense. Everything is entwined.”