Jackson hurt himself while jumping a route and planting his foot awkwardly in the turf. He could put very little weight on his right leg while walking into the team facility.
This comes at the Ravens' thinnest position. Lardarius Webb remains out with a back injury, and Aaron Ross was lost for the season after tearing his Achilles tendon before training camp officially began.
Jackson was considered the frontrunner to win the Ravens' No. 3 cornerback spot. He has had a solid training camp and made an athletic interception in the preseason opener.
This injury is the most significant one on the second joint practice between the Ravens and the San Francisco 49ers.
Six Ravens aren't practicing Sunday: Webb, tight end Dennis Pitta (undisclosed), guard Ryan Jensen (ankle), safety Brynden Trawick (back), guard Will Rackley (head) and nose tackle Terrence Cody (hip).
*UPDATE: Jackson has a minor ankle injury, according to coach John Harbaugh. "He'll be out for some number of days, but it's not a major deal," Harbaugh said after practice.