New England Patriots tight end Rob Gronkowski (87) looks on from the sideline after an argument with Buffalo Bills players during the second half Sunday, Dec. 3, 2017, in Orchard Park, N.Y. (AP Photo/Adrian Kraus)

Rob Gronkowski is one of the most watchable players in the NFL, a combination of cheerful juvenility and giant burliness, with his wood-axe forearms, and hair that looks like it was moussed with last night’s beer.

If there was a mean streak in him, it was undetectable before Sunday, when he suddenly helmet-stabbed a defenseless player lying facedown on the ground.

Nevertheless, Gronkowski should have been suspended for multiple games.

The NFL has made examples out of lovable poster boys before, for far less. All of the stupidity and incoherence of the league’s so-called “disciplinary system” has come home to roost on this one.

What are Roger Goodell’s broad powers and harshest penalties for, if not this? In Deflategate, Tom Brady received a four-game suspension over a whiff of air in footballs. In Bountygate, players were given suspensions ranging from six games to an entire season for targeting. Yet Gronkowski got just one game for an act of violence so out of bounds that if it had occurred on a city street, he would have been handcuffable.

The optics are awful. There is no whitewashing what Gronkowski did as boyish foible. There is no characterizing the NFL’s disciplinary “system” as anything but an unprincipled mess.

The hit Gronkowski leveled was breathtakingly gratuitous. The New England Patriots were leading 23-3 late in the fourth quarter when rookie cornerback Tre’Davious White outmuscled Gronk to make a leaping interception. White was lying prone on the sideline, when here came Gronkowski at full speed, that rock-quarried body fully weaponized. He delivered a forearm to the back of White’s helmet, driving it into the ground so hard that White is being treated for a head injury. So much for “culture change” in the NFL, and its concern over the concussion crisis.

Gronkowski says the hit was simply a matter of over-competitiveness, after a lot of unpenalized physical exchanges with White throughout the game.

“First off, I definitely want to apologize to No. 27. I’m not in the business of that,” Gronkowski said after the game. “I was just really frustrated at that moment. . . He was trying to push me a little bit … I just don’t understand why there wasn’t a flag a couple times in the game … I mean, like what am I supposed to do? And then they don’t call that, I mean. It was just frustration, and that’s what happened.”

The explanation is disingenuous nonsense.

It’s important to make two things clear: The hit came after the whistle – long after it – and it came with White lying out of bounds. It’s one thing to make an illegal hit in the course or continuation of play. To a certain extent, all players sign up for the potential of injury before the whistle. This was in a different category altogether. It was deliberate, vengeful and intended to do harm.

All of Gronkowski’s excuses pale in the face of the replay, which shows it was quite simply one of the latest and most deliberately vicious hits you ever saw, straight into the back of a facedown opponent. This was no fly-off-the-handle moment. It was a calculated act, pure ego-driven retaliation against an unwary opponent. And worst of all, it was a shot to the head, when Gronkowski knows full well that concussions can lead to long term neurological problems, even a fatal disease.

Yet the NFL declined to throw the book at Gronkowski, settling instead for the minimum it could levy without appearing totally callous.

What’s the message? The rules have been rendered totally meaningless, and precedents don’t mean a thing. Goodell will fight to the brink of the Supreme Court to enforce a suspension on Brady over the inflation of a football. He will play cop on domestic violence cases, for the sake of image and quieting public outcry. But in the case of unacceptable violence on the field — the one issue on which he should exercise his full authority — he backed down.

Deflategate, according to Goodell, was about conspiring to subvert the rules. But what rule could matter more than one governing violence after the whistle? If Gronkowski’s hit is no big deal, if it’s worth only one game, then which rules do matter?

Just because White was wearing a player’s uniform doesn’t mean he wasn’t assaulted. He could not have been more defenseless. And it hardly matters that Gronkowski and White had a physical scuffle all game.

“What was I supposed to do?” Gronkowski asked. Manage himself, that’s what. Gronkowski is not paid $54 million over six years to lose control, but to maintain control — over his speed, his body and his temperament. The NFL is not a game of violence, but rather a game of real violence averted. All of its rules are geared toward that restraint. Otherwise it’s just fighting.

Other players have been handed multigame suspensions for less provable and less ugly offenses. Goodell continually cites “conduct detrimental” to the league in his disciplinary actions. This was one instance in which the commissioner would have been justified in using his unfettered draconian powers to issue a multigame suspension.

Trouble is, he didn’t have the stomach for this particular issue. It’s hard to think of anything more detrimental to the NFL, anything more undermining of league integrity.