SANTA CLARA, Calif. -- As the first quarter wound down in his second NFL game, San Francisco 49ers running back Isaac Guerendo trotted on to the field, stepped into the huddle and listened intently as quarterback Brock Purdy called the play.
The call was a run for Guerendo and his first chance to make an impression. Guerendo took the snap, bounced to his left and quickly cut back to his right where he was almost instantly tackled for no gain.
As first NFL carries go, it wasn't particularly memorable and Niners coach Kyle Shanahan said if Guerendo hadn't rushed into his cut, he'd have gained a maximum of three yards. But it offered a quick and easy lesson about the patience necessary to succeed as a running back in Shanahan's outside zone heavy offense.
"It takes guys some time," Shanahan said. "If you've got the right stuff, the more you get reps, the more you can adjust to it. How hard you've got to hit stuff, how quick those holes close, how when there is a hole how you have to hit it full-speed and can't hesitate at all or it closes like that."
That early in the season, time was undoubtedly on Guerendo's side. With Jordan Mason handling the bulk of the carries until Christian McCaffrey would return from Achilles tendonitis, the Niners could sprinkle Guerendo in here and there to get some needed reps without relying on him to lead the way.
Likewise, Guerendo could continue watching and learning from McCaffrey, Mason and running backs coach Bobby Turner without the pressure of having to be a focal point in the offense.
Fast forward to this week and the time for waiting is over.
McCaffrey was placed on injured reserve again, this time with a tear in the posterior cruciate ligament in his right knee. To make matters worse, Mason has a high ankle sprain and is also expected to go on IR. Neither is expected to return this season, which means Guerendo will make his first NFL start on Sunday against the Chicago Bears (4:25 p.m. ET, Fox) with the opportunity to establish himself over the season's final five weeks.
"He's gotten better throughout the year," Shanahan said. "I think he's ready for this."
The 49ers used a mid-round pick -- fourth round, No. 129 overall -- on Guerendo in April's NFL draft. The idea was to add the 6-foot, 221-pound speedster (he ran the 40-yard dash in 4.33 seconds at the scouting combine) to a crowded room that already featured McCaffrey, Mason and Elijah Mitchell.
Ideally, the 49ers wouldn't have needed Guerendo to offer much more than kick return ability this season while he learned the ropes from McCaffrey, Mitchell and Mason. If that went well, Guerendo would be well-positioned for an expanded role in 2025 when Mitchell is scheduled for unrestricted free agency and Mason will be a restricted free agent. Injuries to all three of those backs (a hamstring injury ended Mitchell's season before it began), have given Guerendo occasional opportunities beyond that lone carry in Minnesota.
Guerendo has flashed the home run ability that made him so appealing to the 49ers coming out of Louisville -- he has 42 carries for 246 yards and a pair of touchdowns. He has five runs of 10-plus yards, including a 76-yarder against Seattle and a 14-yarder against Dallas to put those games away.
Guerendo's 5.9 yards per carry is the fourth-best mark among any of the 67 running backs with at least 40 carries this season. Suffice to say, he has embraced the outside zone credo "slow to, quick through," which means to be patient in the backfield but once you see the hole to hit it as hard and fast as possible.
"I think that's one of those things that just comes with reps," Guerendo said. "The more reps you get, the more you get the feel for it and the more I've gotten, I think I've caught onto that. So that's something that's helped me and will continue to help me."
If Guerendo can stay healthy, the next five games should provide plenty of those much-needed reps. In many ways, Guerendo is still a bit of a running back neophyte.
Before he arrived at the University of Wisconsin as a freshman, Guerendo hadn't played running back since he held the job for his fourth-grade peewee team. After switching from receiver to running back in Madison, he started one game in the backfield in five seasons split between Wisconsin and Louisville.
Hamstring injuries limited Guerendo to just 12 carries in his first two years with the Badgers and a foot injury cost him nine games in 2021. He never had more than 64 carries across four seasons before transferring to Louisville.
In his lone season with the Cardinals, Guerendo played in 14 games with 819 yards on 132 carries with 11 touchdowns. The sample was small but Guerendo's success in an offensive scheme with similarities to what the 49ers deploy was appealing for Turner, Shanahan and run game coordinator Chris Foerster.
That blazing speed and upright running style didn't hurt, either, as Foerster saw striking similarities between Guerendo and another former Niners running back who wore No. 31, Raheem Mostert.
"I'm not putting him in that category yet, but he really does (run similarly)," Foerster said. "He's got a lot of developing to do. And that position is a hard, tough position. But he's nothing but a positive upside... He could be your lead dog, but he's got that extra gear we've been looking for in a change between, Christian's playing fast, JP's playing fast, but he's got a little bit something different from those guys."
Earlier this week, Guerendo and McCaffrey shared a brief moment. Guerendo told McCaffrey he was praying for him to have a speedy recovery to full health. McCaffrey told Guerendo if he needs anything, he will be around and happy to help.
While there's a long way between here and there, it's at least a possibility that McCaffrey and Guerendo will be the Niners' top two backs in 2025. How that looks will depend on how Guerendo fares now that he's finally set to take on the role he hasn't had since elementary school.
"Isaac's done a great job," linebacker Fred Warner said. "We've used Isaac a ton this season, but now it's his turn to really take a hold of that position."