Antonio Brown has been fired by his powerful NFL agent Drew Rosenhaus -- who says he won't work with the embattled WR again until he gets professional help.

Rosenhaus -- who has worked with Brown for years -- sent a letter to the NFL Players Association saying he wants to work with AB again, but strongly believes Antonio needs counsel first, this according to ESPN's Adam Schefter.

Rosenhaus -- whose client list includes stars like Tyreek Hill and Rob Gronkowski -- had been sticking by AB's side through all the drama with the Steelers, Raiders and Patriots ... but now it's clear he's reached his breaking point.

In fact, Brown recently had a workout with the New Orleans Saints but reportedly blew it -- bringing an entourage and camera crew despite being told NOT to do that.

Now, it seems he's on the outs with the NFL -- especially with the league investigating allegations of sexual assault, threats and more -- and Rosenhaus knows AB needs serious help if he expects to get another shot in the league.

Earlier this week, the Hollywood Police Dept. in Florida announced they were severing ties with the WR after he berated officers and his baby mama on video -- calling Brown a terrible role model.

Meanwhile, AB is working on a second career as a boxer -- he's in talks to fight Logan Paul in April ... but it seems Rosenhaus won't be negotiating the contract.