Martin, a third-round draft pick, was carted off the field early in the fourth quarter.
“We don’t have any indication how serious it is,” 49ers Coach Jim Harbaugh said.
Martin wrote Sunday night on Instagram: "What don’t kill me, will only make me stronger! I’ll be back better than ever before, thank you everyone for the well wishes!! Love all ya’ll. #GRIND #49ers"
Martin dislocated his left kneecap in USC’s regular-season finale against UCLA last season. He was carted off the Coliseum field and did not play in the Las Vegas Bowl.
Martin sat out the first week of the preseason recovering from an ankle injury. Harbaugh praised Martin after he made his debut against the Denver Broncos.
Martin was competing to back up Daniel Kilgore.
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