Struggling '49ers offensive line looks to make adjustments in a hurry for final preseason game

San Francisco 49ers quarterback Blaine Gabbert (2) passes as running back LaMichael James (23) blocks San Diego Chargers cornerback Chris Davis (42) during the third quarter of an NFL preseason football game in Santa Clara, Calif., Sunday, Aug. 24, 2014. (AP Photo/Mathew Sumner) (The Associated Press)

San Francisco 49ers quarterback Colin Kaepernick (7) runs against San Diego Chargers defensive end Kendall Reyes, bottom, and defensive end Corey Liuget (94) during the first quarter of an NFL preseason football game in Santa Clara, Calif., Sunday, Aug. 24, 2014. (AP Photo/Mathew Sumner) (The Associated Press)

SANTA CLARA, Calif. – San Francisco 49ers rookie center Marcus Martin suffered a left knee injury that isn't considered season-ending, yet still a blow to the depth of a struggling offensive line.

The third-round draft pick was carted off the field with 11:18 remaining during a 21-7 preseason victory Sunday against San Diego.

San Francisco coach Jim Harbaugh said it doesn't seem to be an ACL injury, and it appears Martin might be able to return later in the season.

Martin already dealt with an ankle injury that sidelined him early in training camp. The 70th overall pick out of Southern California, Martin played 38 games — with 33 starts — over three seasons with the Trojans.

The 49ers are looking to make some quick adjustments on their struggling offensive line, which showed its vulnerabilities in pass protection as quarterback Colin Kaepernick took several hard hits and was constantly pressured Sunday.