Cash, in his second year out of USC, was injured Saturday night in the Lions' preseason contest against Cleveland Saturday night. He has been placed on injured reserve.
Coach Steve Mariucci told the Detroit Free Press that Cash probably will not have the surgery for several weeks.
"We're going to try to let his MCL heal," Mariucci told the paper. "The way it sounds after talking to Dr. (Kyle) Anderson, it sounds like there's going to be a few weeks healing time for his MCL before they consider surgery on the ACL."
The 2002 sixth-round draft pick started 12 games last season and was ranked first overall in tackles among NFL cornerbacks.
His injury is the latest for the Lions' secondary.
Dre' Bly, signed during the offseason, may be back from an ankle sprain in time for the regular season opener Sept. 7 against Arizona.
Also, cornerback Chris Watson underwent a surgical procedure last week to alleviate discomfort in his back and will be unavailable for several weeks.