Uncle Drew's beef with Cs fans has been going on ever since his departure from Beantown in 2019 ... and another chapter was added to the rivalry as he made his way to the locker room following Dallas' 105-98 Game 2 loss at the TD Garden.

The home fans were clearly giving it to Irving ... which prompted him to throw his hand up to signal the Mavs will take care of business when the series moves to Dallas for the next two matchups.

It shouldn't come as a surprise -- Celtics backers haven't forgiven Irving for signing with the Brooklyn Nets in free agency a few years back ... which was evident when some folks beat up a blow-up doll and chanted "f*** Kyrie" prior to Game 1.

The 8-time All-Star expected his former team's fans to express their displeasure with him after his unceremonious exit from the franchise ... but when reflecting on his time in Boston, he admitted he "wasn't his best self" while sporting the Green and White.

"I know sometimes in sports, it’s literally about the end goal and result in what you accomplish, and that’s one thing," Irving said. "But we’re still human. At the end of the day, I wasn’t my best self during that time. When I look back on it, I just see it as a time where I learned how to let go of things and learned how to talk through my emotions."

Irving hasn't been his best so far in the series ... scoring 12 and 16 points in the first two games -- which certainly puts a smile on the faces of Boston fans.