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.”Nick Bosa's right quad is smaller than his leftMatt BarrowsHow 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.