says there's plenty of heat left on 's fastball ... telling TMZ Sports his former head coach is still every bit the "powerhouse" he used to be.

"I think, 100 percent, he's still the great Bill Belichick," the Patriots legend said. "No doubt about that."

Of course, it's all come into question recently ... after Belichick struggled to find success in New England following 's departure.

In his four seasons without the GOAT under center, Belichick recorded just a 29-38 record .

.. and only made one playoff appearance.

Now that of Foxborough, though, Gronk says he believes Belichick can prove once more he's still the best ever to put on a headset ... telling us if he gets a hold of a roster with stars on it, watch out.

"If he goes to a team with great players with great talent, he will be successful," Rob said, "because he's still the powerhouse coach that we all have seen throughout the years."

For now, Belichick seems more likely than not to end up in Atlanta ... he already interviewed for the Falcons' vacant job earlier this month -- and, according to NFL Network's

, he'll have a second meeting with the team's brass this weekend.

As for The Hoodie's replacement in New England ... Gronkowski had nothing but good things to say about Jerod Mayo -- telling us all he's heard so far about his former teammate is that he's become a helluva a leader on the sidelines.

There's much more with Gronk ... we also talked to the NFL legend about his upcoming Super Bowl party, , at Encore Beach Club on Saturday, February 10.

The 5,000-person strong pool party/music festival will have all kinds of entertainment, including Afrojack and DJ Irie, in addition to other surprise musical guests.

In past years, Rob's brought some of his famous to the party ... like Travis Kelce, Julian Edelman, Guy Fieri, Deebo Samuel, Johnny Damon, Camille Kostek and even Bill Belichick!

Here's your chance to party with Gronk ... don't drop the ball!