The San Francisco 49ers knew they'd miss their five-time Pro Bowler Nick Bosa as they took on the Jacksonville Jaguars, but they might not have known how much. There was plenty of blame to go around for the Week 4 debacle that coincided with Brock Purdy's return to the starting lineup, but the impact of Bosa's absence stands tall among the issues as something that is not easily cleaned up.
Over the first couple of weeks, with a healthy Bosa, the Niners defense was instrumental in sealing wins. Case in point was Bosa himself, singlehandedly snatching victory in Week 1 with a critical strip sack of Sam Darnold, going on to recover the fumble and end the Seattle Seahawks' threat.
Robert Saleh's defense requires the ability for the front four to consistently get pressure and get up the field, regardless of run or pass. That puts pressure on the linebacker to clean up in the run game, and the secondary to rally to cut down receivers before significant damage can be done against the pass.
The execution of that philosophy is so much easier with an elite edge rusher who can win on his own, regardless of the matchup, and wreak havoc.
Without him, some painful truths have been exposed with the Niners defense.
These 3 painful truths about 49ers defense are now evident in Nick Bosa's absence
Bryce Huff isn't a suitable replacement for Nick Bosa as a pure pass-rusher
Bryce Huff broke out with the New York Jets with Saleh as his head coach, emerging as one of the best situational pass-rushers in the league. With explosive speed and great bend, the former undrafted free agent exploded to become a 10-sack player in 2023 with the help of Saleh's guidance.
Undersized and a liability in the run game, Huff isn't a straight replacement for Bosa, but there was hope his prowess bending the edge could at least pick up a lot of the slack in the pass rush, even if it would be much more explosive with Bosa bookending him on the line.
Huff had looked strong with Bosa on the other side for the first few weeks of the season, but against the Jaguars, San Francisco failed to record a sack. Huff, in particular, was invisible for most of the action, registering just two pressures.
Here's the rub with Huff: He needs another elite rusher to play off. With the Jets, he had the benefit of Quinnen Williams creating havoc in the middle, as well as a deep and varied attack from the other edge, which opened up opportunities for him. As the main guy to lead the charge, he falls short.
That will create some issues for Saleh and the team moving forward.
Nick Bosa's impact in run defense can't be overstated
Jaguars running back Travis Etienne sliced and diced the 49ers defense for 124 yards on 19 carries. The big blow came when he ran off tackle to the left and cut upfield for a breakaway 48-yard touchdown.
That run, of course, came right where Bosa is typically hanging out, and while he's first and foremost known as a terrifying pass-rusher, Bosa is a nightmare in the run game as well.
With PFF run defense grades of 81.1, 81.9, and 79.9 from 2022 through 2024, respectively, Bosa has always set a strong edge and been remarkably consistent in his run defense.
Edge rushers might not be the key to stopping the run in Saleh's system, but the prevalence of outside-zone attacks as the league copies Kyle Shanahan's offensive scheme makes sturdy defense on the edge a necessity.
Unfortunately, the 49ers don't have anyone with the chops to adequately fill Bosa's shoes in this regard.
Robert Saleh's conservative philosophy stands little chance of holding up without Nick Bosa
Saleh might be known for his "All Gas, No Brakes" mantra, but in reality, aside from having his linemen pin their ears back every play and focus solely on one gap, his defense is actually pretty vanilla.
Blitzes are a rare occurrence, with Saleh counting on his four down linemen to win without any additional help.
On top of that, Saleh enjoys using a Cover 3 shell that is designed not to get beat over the top and instead attempts to force the check down from the opposition. From there, he counts on his linebackers and secondary to rally to ball carriers and make solid tackles that minimize yards after the catch.
While that means, if executed properly, few big plays are ever given up, it also means the defense rarely makes any game-changing plays themselves. So far in 2025, the Niners defense ranks 23rd in the league in turnover percentage at 7.1. They're also one of only three teams yet to record an interception.
The good news is that Saleh has shown he can adjust. In 2020, there was a rash of injuries, including a season-ending injury to Bosa in Week 2. The sheer number of injuries forced Saleh to adjust, blitzing far more often than his norm and playing more aggressive coverages in an effort to mask the deficiencies of the personnel to run his scheme as intended.
Ultimately, the 49ers would finish that year ranked fifth in total yards allowed per game at 314.4, while posting about a league-average performance, allowing 24.4 points per game. Considering the quantity and severity of the injuries, those were some admirable results, and the ability to revamp his scheme on the fly was a big reason he landed the head-coaching gig in New York the following January.
And that's the silver lining. Saleh has already shown an ability to mold his scheme to the talent he has, rather than try to force the talent to fit the scheme.
Now, without Bosa, he'll have to get creative again.