49ers' Jerick McKinnon officially on IR, says he'll help Alfred Morris and Matt Breida

SANTA CLARA, Calif. -- Two days removed from a season-ending knee injury that will prevent him from getting his first chance to enter a season as a starter, San Francisco 49ers running back Jerick McKinnon's biggest disappointment was that he won't get to play against his former team on Sunday.

"I was more devastated when it happened, just the feeling knowing that it wouldn't be all right for Week 1," McKinnon said Monday. "This game probably meant a lot [more] to me [than] maybe anybody else, just going against my former team and stuff like that. So, when it happened and I felt it, I would probably say that initial thought of like 'Dang, I'm not going to have a week to get my knee right' was probably the worst feeling.

"And then once they told me what the injury was, it just kind of confirmed it."

McKinnon suffered a torn ACL in his right knee on the final snap of Saturday's practice as the Niners began preparations for Sunday's game against the Minnesota Vikings. That game had long been circled on McKinnon's calendar after he played four seasons for the Vikings before signing a four-year, $30 million deal with the Niners in March.

Because McKinnon's injury came without contact -- and because of how awkwardly he landed -- the Niners had a bad feeling about it almost immediately. So, too, did McKinnon, who said he was simply running and making a cut before he felt his foot get stuck a little bit and his knee twist.

Hours later, an MRI confirmed what McKinnon and the Niners feared. San Francisco officially placed McKinnon, along with safety Marcell Harris(hamstring), on injured reserve Monday afternoon. The Niners signed offensive lineman Matt Tobin and defensive back Antone Exum Jr. to fill those roster spots.

McKinnon's knee injury came on his first full day back at practice after working back from a right calf strain that had kept him out most of the preseason. McKinnon said he's unsure if that injury had anything to do with the knee but doesn't believe he came back too soon.

"I don't feel like I rushed back," McKinnon said. "That was the plan that was intact, that's what we went with. I felt good at practice. It just so happened, I made a cut and I tore my ACL. That's all it was."

Since the injury, McKinnon said he heard from teammates old and new, including former Vikings teammate Adrian Peterson, who came back from a similar injury in eight months and went on to rush for 2,097 yards on his way to 2012 league MVP honors.

McKinnon will have surgery to repair the ligament once the swelling has gone down, though a date has not yet been set.

In the meantime, McKinnon, who was in good spirits considering how recent the injury is, plans to be around the team as much as possible and said he will be the Niners' "biggest fan, biggest cheerleader."

"You can't sit here and be down about it the whole time or feel sorry for myself because that's not going to do nothing but just set me back even more," McKinnon said. "I'm here to root these guys on, make sure they stay straight and we all hit our team's goals that we've been talking about since I got here."

McKinnon also plans to serve as a sounding board for his replacements, which will mostly consist of veteran Alfred Morris and second-year pro Matt Breida.

Like McKinnon, Breida returned to practice on Saturday from an injury (shoulder) and is considered full go this week. Both Morris and Breida participated in Monday's practice.

While nothing definitive has been said to either about how much their work load will increase without McKinnon, Breida and Morris figure to each get plenty of work depending on situations and matchups.

Breida was the primary backup to Carlos Hyde in 2017 and finished with 465 rushing yards with a 4.43 yards-per-carry average and two touchdowns to go with 21 receptions for 180 yards.

During the offseason, Breida said he made improved pass catching one of his top priorities, something he -- along with fullback Kyle Juszczyk -- will have to do more of without McKinnon.

Breida, who has known McKinnon since he was a freshman teammate of his at Georgia Southern, said he was already set for an expanded role even before McKinnon's injury.

"I feel like they're going to use me a lot in the offense this year more anyway," Breida said. "So, I've been ready. [McKinnon's] going to help me a lot. Alfred's knowledge of the offense can help me a lot and me and Al and Raheem [Mostert], we're all going to get the job done."

Morris offers a steady, veteran presence in McKinnon's absence. Entering his seventh NFL season, Morris is familiar with coach Kyle Shanahan's offense after spending his first two years with Shanahan as his offensive coordinator in Washington.

Those were also Morris' two best NFL seasons, as he rushed for a combined 2,888 yards and 20 touchdowns while averaging 4.72 yards per carry. He has 57 receptions in his career.

Breida offers more speed but Morris is a bigger, more physical option who could be used more in short-yardage and red-zone situations. After waiting for what he called the "right opportunity," Morris signed with the 49ers on Aug. 14. Although he's played for Shanahan before, he hasn't been in the offense since 2013, so there is still an adjustment period, mostly related to things like terminology and some concepts.

"It's not second nature yet, and that kind of bothers me," Morris said. "Because when you think, you're not playing as fast. You're slow off the ball, you're thinking about, OK, what am I supposed to do? I'm supposed to protect. So just really working on getting the playbook and taking those reps in practice so it becomes second nature. Because the quicker I can think, the faster I get to play. Right now, I'm still kind of in the stage of still learning, to solidify it and getting that confidence. But I think I'm getting to that place that I can carry a heavy load."