I know there is always speculation on whether an impending FA will re-sign with their current team; however, if you are referring to Gorman's, "Kyrie Irving staying with Celtics long term is not a given," comment, he was referring to whether the Cs will want to re-sign Irving (if injury issues continue), not Irving leaving on his own volition.
That being said, I would have to put the chances at 95% if he makes it all the way to the end of his current contract without being traded. He may take a glance at other teams, but he will make the most money in Boston and has a ready-made championship team that will be good for at least a decade.
I do not think he will sign an extension this off-season, but it sure would be awesome if he did!