Gronk says it's all possible after Edelman's retirement from the Patriots ... explaining he believes there's a "69% chance" Julian's a Buccaneer in 2021!
The future Hall of Fame tight end made the revelation at LAX on Monday night -- just hours after Julian hung up his Patriots spikes -- saying he believes his former New England teammate ain't done with the NFL yet.
"He'll be back," Gronk said of Edelman. "He'll be back. Yeah."
Edelman, though, didn't seem so sure of that on Monday ... in his retirement announcement, Julian explained injuries just won't let him go any further with his football career.