Boss Joe Royle told the club site: "The ligament is torn completely and that means Jay is out for the season really.
"It's a real blow for the lad and he will need surgery.
"We will talk to Blackburn and find out whether they want their specialist to carry out the operation or our people.
"It's another knock for us. We have to deal with it but it's certainly a blow for us and Jay."
Physio Dave Williams said: "Unfortunately the ligament is detached and has come away from the bone.
"Although I wouldn't rule him out completely for the season, it's looking likely that will be the case."
McEveley had been called up to a Scotland futures squad by Walter Smith and the results of the scan will have come as a shock to the young full-back, who was only expected to be sidelined for around a month, as reported by ITFM on Thursday.
Jay's inevitable departure from Prtman road, however, could see Matt richards revert back to a Defensive role, or give a regular return to the starting line-up to 35 year-old dutchman Fabian Wilnis.