SANTA CLARA, Calif. -- Given his status as a veteran and one of the team's best players, San Francisco 49ers running back Christian McCaffrey was always unlikely to play much, if at all, in the preseason.

If there was a chance for that, however, it no longer exists. McCaffrey did not participate in practice Tuesday after missing the team portion of drills Sunday because of a strained calf. Coach Kyle Shanahan said McCaffrey is likely to miss a couple of weeks of practice and is all but certain to miss the team's three preseason games.

"He did it a couple days ago," Shanahan said. "It's all right. He didn't pull it or anything, but you guys probably won't see him this preseason."

A two-week timeline would give McCaffrey, the NFL's reigning Offensive Player of the Year, plenty of time to return to practice before the Sept. 9 season opener against the New York Jets on "Monday Night Football." Coming off a season in which McCaffrey had a career-high 417 touches (including playoffs), the Niners have been giving him regular rest during this training camp.

Under the normal schedule, McCaffrey was due for a veteran's day off along with end Nick Bosa on Tuesday, but although Bosa watched practice from the sideline, McCaffrey was in the midst of rehab and out of view during the session.

In the opening days of camp, McCaffrey noted that training camp is important for him as a means to prepare for the hefty workload he takes on in the course of a season.

"For me, it's put my body, mind and soul in the best position to play every snap," McCaffrey said. "And then coaches got to be coaches, whatever they think is best, but I know for me, I prepare to play every snap so I'm ready to go."

McCaffrey isn't the only Niners running back dealing with injury. Elijah Mitchell has missed the past two practices because of a strained hamstring.

"He didn't pull it, but we'll be safe with him," Shanahan said. "So, he's going to miss probably a week or so."

Mitchell did some light running on the side during Tuesday's practice alongside yet another injured Niners running back, rookie Isaac Guerendo. Guerendo pulled his hamstring early in camp and has missed most of the team's practices.

Without McCaffrey, Mitchell and Guerendo -- and with the preseason opener Saturday against the Tennessee Titans -- the Niners signed running back Matt Breida. Breida spent his first three seasons with the Niners after they brought him in as an undrafted free agent in 2017.

Breida participated in Tuesday's practice with the Niners' three other healthy running backs: Jordan Mason, Cody Schrader and Patrick Taylor Jr. To make room on the roster, the Niners placed receiver Malik Turner on injured reserve.