Coach Jim Harbaugh said during a news conference Thursday that he didn't yet know the severity of Culliver's knee injury, but the cornerback was "at Stanford getting an MRI."
Culliver was sprinting downfield in a one-on-one punt coverage drill with rookie Marcus Cooper when he crumpled to the ground, according to Barrows. Grasping his left knee, Culliver was wheeled away to the 49ers' training room. Eric Branch of the San Francisco Chronicle tweeted that Culliver -- being comforted by general manager Trent Baalke -- "was in tears" as he left the practice field.
Culliver, in his third season, isn't tabbed as a starting corner in San Francisco's secondary, but he saw plenty of snaps alongside Carlos Rogers and Tarell Brown in 2012. He struggled mightily in the Super Bowl -- burned more than once by Anquan Boldin -- and caused a firestorm after making insensitive remarks about the prospects of an openly gay player in the NFL.
Still, Culliver graded out as the 29th best corner by ProFootballFocus and -- if able -- figures to play a large role on the outside against three- and four-receiver sets in 2013.