McCann, who missed a large chunk of last season after suffering two knee injuries, suffered a fresh setback during the Clarets' pre-season tour of Singapore.
And following a fresh diagnosis, the 23-year-old Irish midfielder has gone under the knife for a full, reconstructive operation.
Boss Brian Laws said: "Chris needs a full reconstruction of the cruciate ligament. It is devastating news, not only for the football club but particularly for a player who has had a dreadful time trying to overcome anterior cruciate damage from over a year ago.
"Chris has had niggles consistently over a period of time and it got to a point where we needed a second opinion. Unfortunately, that opinion is that he needs a full reconstruction and that we are now going to lose him for a minimum of six months.
"We must make sure it's clear to everyone that this is not career threatening and the plus is that he will make a full recovery and eventually be 100 per cent right and not looking at percentages on whether he will be fit or not."