If Sewell, a true freshman, misses the full six weeks, he could return for the Pac-12 championship game (if Oregon qualifies) or a bowl game.
Sewell has started each of the first six games, becoming the first Oregon freshman offensive lineman to start a season opener since 1997. He sustained the injury to his right leg in Saturday's 30-27 overtime win over Washington.
"Penei is a relentless competitor and I have no doubt he'll return better than ever," Cristobal said in a prepared statement.
Oregon moved starting right tackle Calvin Throckmorton to the left side after Sewell's injury against Washington. Junior George Moore is listed as Sewell's backup on Oregon's depth chart, but backup right tackle Brady Aiello could be a stronger option to play either tackle spot. Aiello started 10 games at left tackle in 2016.
Sewell had been the left-side protector of Oregon junior quarterback Justin Herbert, who threw two touchdowns against the Huskies on Saturday and was playing in front of a large gathering of NFL scouts and executives that included Denver Broncos GM John Elway.
ESPN rated the 6-foot-6, 345-pound Sewell as the No. 6 guard, No. 129 overall player and No. 1 prospect from Utah in the 2018 recruiting class.
The Associated Press contributed to this report.