Celtic are optimistic John Kennedy will make a complete recovery after the defender underwent a fourth operation on his damaged left knee.
Kennedy only recently returned to training with the Celtic first-team
The 22-year-old suffered his latest setback last week, twisting his knee in a training ground incident.
A club spokesman told the club website: "John has undergone some surgery which we had hoped to avoid - but which could have been needed in any case.
"Dr Steadman is hopeful of him making a full recovery and playing long-term."
Kennedy, who agreed an extended contract at Celtic recently, has not played since March 2004 when he was injured playing for Scotland against Romania.
He made the visit to Colorado-based Dr Richard Steadman with club doctor Roddy Macdonald and is to remain in Vail for the next few weeks to continue his rehabilitation.
A member of Celtic's medical staff will travel to oversee the later stages of his rehabilitation before accompanying the centre-half home.