Riley told reporters in Norman that Norwood sustained a non-contact injury. A source told ESPN's Mark Schlabach that Norwood injured his knee during practice Monday.
Norwood is one of the most experienced and versatile players in the Oklahoma secondary, starting the last 19 games. He worked at cornerback, safety and nickelback in the spring and was expected to be a key player in the revamped defense new coordinator Alex Grinch is installing.
Last season, Norwood had 58 tackles, two tackles for loss, one interception and five pass breakups. His interception came at Oklahoma's 1-yard line in the final minute of its 39-27 victory over Texas in the Big 12 championship game.
The Sooners open the regular season against Houston on Sept. 1 (7:30 p.m. ET, ABC).