Keeping the depth chart afloat is turning into a full-time grind for the 49ers. Christian McCaffrey’s heavy usage and Patrick Taylor’s season-ending shoulder injury have spun the RB room into a revolving door. And when Isaac Guerendo came back from his shoulder injury, it looked like he’d be ready for a bigger role.

And today, in the 49ers’ final game against the Chargers in the preseason, was the perfect opportunity for him to prove he’s ready for increased snaps. But things never pan out that seamlessly, do they?

Yeah, in the first minute of the first quarter, he went down diving in for a fumbled catch, and you just knew something was wrong by the awkwardness of that dive. And yeah, we were right. He stood up, holding his shoulder, and walked away to the sidelines. That shoulder never recovered.

This came less than a week after Guerendo was back to full-go in team drills and special teams, with ESPN’s Nick Wagoner noting he was “heavily involved” and trending toward the preseason finale. He’d already sat out the first two games with that same shoulder, so Saturday night was supposed to feel like a milestone… until it didn’t.

But there were signs that he had never fully recovered. Guerendo had already been tagged as “out a few weeks” earlier this month, but one medical observer pointed out he was sporting a right-shoulder brace during his ramp-up. Yeah, maybe playing him wasn’t the right idea.

The worst part? He was pencilled in for more snaps this year. Even with Christian McCaffrey as the headliner, the 49ers value a speed-and-power changeup who can stress the edge and chip in on returns. Guerendo showed that mix as a rookie: 84 carries for 420 yards (5.0 YPC) with 4 TDs, plus 15 grabs for 152 yards and 6 kick returns for 163. Decent numbers. Enough of a case to earn a more pivotal role this season.

His speed was supposed to be Kyle Shanahan‘s weapon. Isaac Guerendo ripped a 4.33 in the 40 at the 2024 combine. It’s the kind of gear that gives Shanahan a different wrinkle alongside McCaffrey’s do-it-all toolbox. But this isn’t just about him. In an already thin backfield, him being sidelined would make Kyle Shanahan reach for an Advil.

Another setback to the RB room

Here’s where it gets messy. The 49ers’ backfield has been hit with injuries beyond Guerendo: Patrick Taylor Jr. went to season-ending IR after a fractured shoulder against the Raiders, and rookie Jordan James had finger surgery that sidelined him for a couple weeks. And of course, the front office tried to bulk up the room.

The response? A direct swing: the 49ers swung a trade for Brian Robinson Jr. this week. He was pegged as a straight-up response to the injury pile-up. It steadies the depth chart behind McCaffrey and, truth be told, felt like insurance the moment the deal went down. Robinson brings a thump-first style and the resume of a back who’s already carried the mail in Washington.

Even so, the context matters. Jordan Mason, the steady change-of-pace back from 2022–24, isn’t around anymore. So if Guerendo misses time, the “speed back” wrinkle has to be reinvented with Robinson and whoever clears final cuts. But it’s not a full-on alarm, just a reminder that Shanahan’s backfield formula depends on mixing and matching skill sets.

So do the 49ers need another back in the room? With Taylor shelved for the year and James still rehabbing, it wouldn’t be a shock if they keep scanning the wire for a one-cut slasher in case Isaac Guerendo’s banged-up shoulder lingers into Week 1.