Belichick, in an unprecedented move in his 16-year Patriots tenure, kept a fully-healthy player in Gronkowski on the bench. There was no chance he was risking the possibility of a major injury to Gronkowski, who is arguably the NFL's toughest matchup for defenses.
That's how much Gronkowski, now in his sixth season, has evolved.
In the season opener on Sept. 10, Gronkowski scored three touchdowns in a 28-21 win against the Pittsburgh Steelers, playing in his first game since Feb. 1 when the Patriots won Super Bowl XLIX.
So it's no surprise Gronkowski has been the centerpiece of the Buffalo Bills' defensive plans this week as the teams prepare to meet Sunday (1 p.m. ET) at Ralph Wilson Stadium, not far from where Gronkowski grew up in Amherst, New York.
"Hands down, you look at it, you're talking about the best tight end in the league," Bills defensive tackle Kyle Williams said. "He's somebody that you really have to account for on every snap."
Bills head coach Rex Ryan has already acknowledged he won't ask one player to consistently defend Gronkowski, unless there is a late addition to the roster with superhuman strength. "He'd have to look like King Kong," Ryan quipped Wednesday.
Ryan didn't seem to be kidding as much when he said there could be as many as three players assigned to Gronkowski, invoking old Patriots game plans against tight ends such as Tony Gonzalez and Antonio Gates when Belichick would devote multiple players to them as if they were a gunner on the punt team.
Such attention would run counter to how the Steelers approached Gronkowski in the opener, at times leaving him unaccounted for despite having stated their intention to be physical with him. Gronkowski walked away with five catches for 94 yards along with the three TDs in the win.
While the Steelers were trying to find a way to stop Gronkowski, the Patriots' coaching staff was looking for any signs his lack of preseason action might require him to be eased into the mix.
Gronkowski played all 61 snaps and even made the heads-up play of the game by sprinting 15 yards to recover a fumble on the goal line in an area surrounded by Steelers.
The performance led Belichick to award Gronkowski the game ball in the locker room afterward, and led receiver Julian Edelman to tell NFL Network "it's like watching an eighth grader play with second graders."
On Wednesday, Ryan assured the Bills won't run into the same problem as the Steelers.
"Obviously you don't just put one guy on this guy; that's been proven," he said, before delivering a zinger to Pittsburgh: "I will say this, it's better to put one guy on him than nobody on him. We'll try to have somebody on him, at least."
For the Patriots, this has become a game within the game with Gronkowski in recent years.
Opponents are now focusing on the 6-foot-6, 265-pound Gronkowski more than ever, so the club has devised ways to make it tougher for them to lock in on him. The opener against the Steelers was a perfect example.
On Gronkowski's momentum-swinging 52-yard catch and run up the right side in the fourth quarter last week, which came on the first play after the Steelers had closed within a touchdown, he started the play in the offensive backfield before motioning into the slot and beating linebacker Bud Dupree.
It was one of four big plays in the game, with Gronkowski aligned in a different spot each time. The Steelers tried safeties against him, as well as linebackers.
This has been the dilemma for defenses, which current Patriots defensive back Tarell Brown experienced in games against New England when he was with the San Francisco 49ers (2012) and Oakland Raiders (2014).
"Before I was here, I just knew he was a helluva player -- 'The Gronk,'" he said. "Being here gave me so much more respect for him, just by the way he works and how he approaches the game."
That approach, teammates like Brown and Edelman say, runs counter to the public perception of the playful, party-seeking Gronkowski. Edelman said Gronkowski is now viewed as one of the team's leaders, with Tom Brady adding, "He's a guy that I have a lot of trust in, there's no doubt about that."
The trust is reflected in Brady throwing an NFL-high 21 touchdowns to tight ends since the start of last season, with Gronkowski accounting for 15 of them. The Patriots' run of success in that area will be tested Sunday, as the Bills have allowed just three passing touchdowns to tight ends over that span (one by the Patriots), tied for second fewest in the NFL.
Williams, now in his 10th year with the Bills, made it clear they've studied Gronkowski closely already.
Now comes the challenge of trying to stop him, and the Bills plan to be physical.
"He can run, he can block, he can catch the ball, run after the catch; does a lot of dangerous things in the red zone," Williams said. "You have to double him up when you can, but you also have to ask guys to hold up when you need other bodies in other places. He's a tough matchup for anybody."
There might not be a tougher one in the NFL right now.