"We're disappointed for Travis," said OU head coach Bob Stoops. "He has worked very hard, and I know that he'll want to get back as quickly as possible."
Javon Harris is expected to see increased time at safety.
"We'll adapt in the meantime," Stoops said. "We've recruited very well and have some excellent athletes in those positions. I'm confident that those players will perform well."
The Sooners can only find solace in the fact that Lewis does not need surgery, but rather a treatment comprised of rest and immobilization, according to athletic trainer Scott Anderson.
Lewis, who decided to return for his senior season instead of heading to the NFL, has led the team in tackles in each of his first three seasons and accumulated 362 so far in his career.
He also has eight picks and six sacks.
Nelson, a sophomore from Dallas, saw action in all 14 games last season and had 21 tackles.
Come on into Sooner Magic Insider for more discussion.