Bill Belichick will be back on an NFL sideline -- after he gets some much-deserved R&R -- according to one of the best players to ever suit up for the legendary Patriots coach, Rob Gronkowski!

TMZ Sports caught up with the future Hall of Fame tight end at his epic Gronk Beach Super Bowl party at Encore Beach Club at the Wynn in Las Vegas, and we asked Rob about BB's future.

"I think it's great he's going to take a year off," Rob told us, adding, "He needs it. he deserves it. And I think he's going to come back next year with some vengeance!"

Of course, after winning 6 SBs in New England, Bill mutually parted ways with the Pats this off-season, and while many around the league expected he'd get a job straight away, it didn't happen.

It's clear Gronk believes his former coach ain't done yet -- he will be back -- and he may be reunited with the superstar TE ... err, WR?

"I'm gonna play for him. I'm gonna be a wide receiver," Gronk JOKED ... "I don't know who he's gonna coach for, but I'm gonna play wideout."

That part's a pipe dream ... Bill coaching a team isn't.

Adding to the likelihood of a return, Belichick's only 15 wins away from becoming the all-time NFL total wins leader as a coach ... which counts regular season and playoff games.

There's much more with Gronk ... who alongside TMZ Sports' own Mojo Muhtadi went absolutely WILD at the party Saturday afternoon.