The announcement came down Saturday from NBA spokesperson Mike Bass, who said .. "The NBA commenced an investigation today into the facts and circumstances surrounding the Dallas Mavericks’ roster decisions and game conduct with respect to last night’s Chicago Bulls-Mavericks game, including the motivations behind those actions."

The reason Adam Silver and Co. are digging into this is because of what seemed to be an obvious move by upper management and the coaching staff to not play their best guys.

Before Friday's matchup against the Bulls, HC Jason Kidd surprisingly announced a number of the best Mavs players were out due to injuries ... including Kyrie Irving. Luka Doncic only played for a quarter and change -- the rest of the game was carried by bench players.

But even with their backup dudes, the Mavs still had a chance to close the game out and secure a spot in the play-in tournament -- which could see them move on further into the playoffs. However, the Mavs somehow blew an 11-point lead in the 4th ... and it appeared they weren't playing their best basketball -- at least that's the consensus on Twitter, anyway.

Adding insult to injury was Cuban's reaction to his team missing the game-tying shot in the final seconds of the game -- which officially ended their chances at getting into the play-in -- and he seemed to be smirking ... further fueling speculation they were tanking on purpose.

Of course, Kidd himself didn't do a great job of masking anything either -- pregame or postgame -- because he essentially said ... yeah, we sat a lot of guys and it was upper management's decision in the hopes of building a better roster for next season. Yikes ...