MELBOURNE, Australia — “Atrophy is real,” says Nick Bosa, who, after all, has had plenty of opportunity to study the phenomenon.
The San Francisco 49ers defensive end boasts some of the bulkiest thighs in the NFL — quads that resemble a pair of gnarled, ancient oak stumps. This year, however, one of those thighs — the right one — is noticeably slimmer than its mate.
That’s because Bosa had surgery in October to repair a torn right ACL, and the ensuing rest and recovery robbed his right thigh of its usual mass.
“I don’t remember (the size difference), but it was drastic after surgery,” he said. “It’s going to take a couple of years to get that last couple of centimeters.”
How does having different-sized thighs affect his game? It doesn’t, he said. And he has the statistics to prove it.
Bosa tore his left ACL in September 2020, then watched as his left thigh lagged behind righty in the run-up to the 2021 season. It didn’t matter. He started every game that season, finished with 15.5 sacks — a career high at the time — and led the NFL with 21 tackles for loss. He made the Pro Bowl and won the 49ers’ Bill Walsh Award, which is given by coaches to the team MVP.
The team is counting on a similar rebound this season, starting Thursday against the Los Angeles Rams. Bosa will play despite missing all but a handful of practices this summer with tendinitis in his right knee.
Defensive end is by far the 49ers’ most worrisome spot as they prepare for Matthew Stafford and a Rams unit that led the league in total offense last year.
Mykel Williams won’t be able to play until Oct. 11 against the Seattle Seahawks after starting the season on the physically unable to perform list. Sam Okuayinonu, who started 12 games last season, broke his foot last week, an injury that threatens to wash out his 2026 season. And third-round rookie Romello Height only recently returned from an ankle injury that kept him out of a week’s worth of practices.
Still, there’s been no panic in the organization to make a trade or bring in a prominent free agent because there’s confidence Bosa will be OK.
After sitting out Monday’s practice, he was back on the field for Tuesday’s session at AAMI Park in Melbourne. Bosa, wearing a compression sleeve on his right knee, took part in the first 20 minutes of practice that were open to reporters and moved around as usual. It’s unknown whether he took part in the full session; the 49ers won’t file a practice participation report until 8:30 a.m. Wednesday Melbourne time (3:30 p.m. Tuesday in California).
“Oh, he’s going out there,” defensive coordinator Raheem Morris said before practice. “He’ll be there. We’re not hiding that.”
Most of the team’s assurance stems from the fact Bosa, who took part in only a handful of practices this summer, has been in this situation before.
He missed virtually all of his rookie offseason in 2019 after suffering a hamstring strain in the spring and a sprained ankle in the summer, yet still managed a sack and three quarterback hits in Week 1 that year. In 2023, a contract stalemate wasn’t resolved until Sept. 6. Bosa played in the season opener four days later, finishing with two tackles and a quarterback hit.
“I always believe that the athlete knows their body better than anyone, and Nick certainly takes that to the nth degree,” general manager John Lynch said earlier this month. “He knows his body extremely well and knows what it takes to have him ready. We saw him one year in a contract year come in here like on a Thursday and play pretty well on Sunday. So we’ve got a lot of faith in his ability to do that.”
Bosa also noted he feels quicker with less mass on his legs.
That was certainly the case in 2021, when he paired his ACL recovery with a revamped diet built on fruits, vegetables, raw fish and egg whites. As a result, his body fat dropped to 9 percent, one of the lowest on the team.
Bosa said this week that his body fat percentage is similar this year, though not quite as extreme as it was after his 2020 ACL tear.
“I don’t know if I’ll be that strict again,” he said. “It was just a little over-the-top in terms of cutting out carbs and stuff.”
He’s also not sweating the disparity in his thigh girth.
Bosa said he could accelerate the muscle building in his right thigh if he wanted to.
“But it’s not really a focus for me,” he said. “I’m not going to change my training just to get size. I don’t train like a bodybuilder — I train like an athlete. I’m not going to just add exercises that aren’t good for being an athlete to add size onto my leg.
“My leg is operating just as good as the other one. That’s all I really care about.”
The more important thing, Bosa said, is he feels faster — like how he felt when he was recovering from his 2020 injury.
“I do feel some similarities to that year, for sure — a little slimmer, lower-body-wise,” he said. “And I’m moving really good. So we’ll see if that manifests itself on the field.”