That's Glen "Big Baby" Davis doubling down on his attack on Kyrie Irving -- saying Kyrie's ankle injury is "karma" for stepping on the Celtics during the playoffs.

Actually, what Glen specifically said was ... "Karma is a motherf**ker!"

Here's the backstory ... Kyrie made a point to step on Lucky the Leprechaun after the Brooklyn Nets beat Boston in Game 4 of their playoff series.

Kyrie famously played for the Celtics from 2017 to 2019 -- but clearly doesn't feel love for the city.

Big Baby, on the other hand, won a championship with the Celtics in 2008 and LOOOOOVESSSS all things Boston.

So, when Kyrie suffered a sprained ankle Sunday while playing the Milwaukee Bucks, Davis sounded off on social media ... saying the whole thing was "karma."

Davis got some pushback on social media -- but he's not fazed, and made a VIDEO going scorched-earth on Kyrie for the disrespect toward his beloved Celtics.

"Everybody mad at me 'cause Kyrie Irving stepped on Lucky and Lucky got his get back," Davis said ... "Don't get mad at me because Lucky got his get back!"

"Karma is a motherf**ker!!"

"You do right by people, people do right by you and I don't care if it's a logo. I don't even care if it's a logo! It means something to me!"

"Like, you know what I'm saying, for example, countries that have flags! You go and step on a logo flag, what is that?! That's disrespect!"

Davis continued, "I played for the Celtics, I'm a Celtic forever!!! So, it don't matter! It don't f**king matter! Stop getting mad because your man got weak ankles!"