According to Kevin Jones of KNBR, Garnett's leg was wrapped and he walked gingerly, suggesting he could be forced to miss some time.
Cam Inman of the Bay Area News Group reported Garnett may require surgery but noted the injury isn't expected to be a long-term issue.
Garnett's left knee was wrapped specifically, and that the injury is not expected to end his season, per Matt Barrows of the Sacramento Bee.
The Niners selected Garnett with the No. 28 overall pick in the first round of the 2016 NFL draft out of Stanford.
He appeared in 15 games for San Francisco last season as a rookie, starting 11 of them at right guard.
The 23-year-old is once again expected to start at right guard for the 49ers in 2017, but his injury could force some shuffling on the depth chart.
With Zane Beadles holding down left guard, the likeliest candidate to start in place of Garnett if he misses game time is Brandon Fusco, who has 64 career starts to his credit with the Minnesota Vikings.
In terms of backups, inexperienced players such as Erik Magnuson and Richard Levy will be tasked with stepping up.