The way that is worded I can't even say what he is saying.
There’s a growing belief of uncertainty that Kyrie Irving will not re-sign with Boston
"There's a growing belief" = "Unspecified persons have expressed that their belief in the likelihood of something is increasing based on recent events"
"of uncertainty that Kyrie Irving will not re-sign with Boston" = "that it is no longer certain that Kyrie will be re-signing with Boston"
Thus:
"Unspecified persons have expressed their belief based on recent events that it is more and more likely that some unspecified people are no longer certain that Kyrie will be re-signing with Boston."
From the context, it makes the most sense to assume that "Unspecified persons" are NBA insiders outside of the Celtics organization and that the "uncertainty" is on the part of the Celtics organization.
In short: Speculation by NBA people that the Celtics arent' sure that Kyrie will resign.
So you gotta ask -- what do NBA people have to gain by suggesting this to Haynes, given that AD just announced he wants a trade and Boston is the first team mentioned in every list of teams that will try to trade for him?