Rosenhaus, though, said it wouldn't surprise him if the now former New England Patriots tight end at some point decided the retirement life wasn't for good enough.
"This is entirely me telling you just a gut-feeling -- I wouldn't be shocked if Rob were to come back," Rosenhaus said.
The agent's opinion comes with a caveat, as Rosenhaus said Gronkowski held firm in the decision to walk away.
"It's not his plan [to come back]," Rosenhaus said. "He told me, 'I'm done. I've had enough. I've won three championships; I just won a championship. I'm healthy, I'm going out on top, I'm feeling good, I'm ready to do some other things.
"He told me he's done. Done. Done. And I tried to talk him out of it. I said, 'Rob, is there anything that I can say to the team, to coach [Bill] Belichick, about practice, training camp, meetings, whatever? He said, 'Drew, I'm happy, I'm content with my decision.'"
Gronkowki's responses leave little room for ambiguity and strongly suggest that returning to football isn't in his immediate or future plans.
He could change his mind, of course, as other players around the league have done in recent years.
As examples, running back Marshawn Lynch stepped away from football in 2016 for a little more than a year before returning; tight end Jason Witten's retired life in a broadcast booth lasted less than a calendar year before he decided to re-sign with the Dallas Cowboys; and cornerback Dominique Rodgers-Cromartie's retirement took five months before he came back and signed with the Washington Redskins.
Whether Gronkowski follows in their footsteps remains to be seen, but for now he finished an accomplished career as a three-time Super Bowl champion, a four-time All-Pro selection and five-time Pro Bowler.
On his nine NFL seasons, Gronkowski totaled 521 catches for 7,861 yards receiving and 79 touchdowns TDs (tied for fifth all time). He also posted prolific numbers in the playoffs with 81 catches for 1,163 yards receiving yards and 12 touchdowns, all representing the most among tight ends.