The second-year offensive lineman is expected to have surgery to "clean up" his knee, which he injured on Friday during training camp practice.
Following the injury, Zane Beadles filled in for Garnett during practices and is expected to now get more reps there.
"I don't know exactly for sure but I know there's some stuff in his knee that we're probably going to have to clean up," 49ers head coach Kyle Shanahan said on Saturday. "I don't think it's anything long-term but it's going to be a little bit of time."
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Garnett figured to be a big contributor for the 49ers in 2017. He was the 28th overall pick in the 2016 NFL Draft. Garnett appeared on Saturday's pre-practice participation report with the knee injury.
The team already lost linebacker Malcolm Smith for the season when an MRI revealed that he suffered a torn pectoral muscle during Saturday's practice.
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