Major League Baseball just peeled the No. 13 off the back of Alex Rodriguez on Tuesday and replaced it with a bull’s-eye.

Might as well stick one to his helmet, too.

By giving Ryan Dempster what amounts to a five-day paid vacation, and the Red Sox the chance to easily maneuver around his absence because of a favorable schedule, MLB didn’t put much of a price tag on using A-Rod for target practice.

This wasn’t just someone acting alone, in the heat of the moment, because Rodriguez gave him a dirty look. This was premeditated, well-planned, team-sanctioned, Fenway-approved target practice.

And that has got to carry a more severe punishment than five games. With full pay. For a starting pitcher who takes the field only every fifth day.

Throwing in the $2,500 fine seemed like someone’s idea of a joke considering Dempster earns $13.25 million this season. But Joe Girardi, who got slapped with a $5,000 tab for too aggressively being the voice of reason, didn’t find the whole matter very funny.

“You just can’t throw at someone because you don’t like them,” Girardi said. “Or you disagree with the way something’s being handled. You just can’t take things into your own hands. It’s not right.”

The Red Sox did, and apparently plate umpire Brian O’Nora was the only one at Fenway who failed to figure that out. Before the Yankees’ visit, Jonny Gomes, Jon Lester and John Lackey took verbal shots at A-Rod, saying how he should have to sit while appealing his 211-game suspension.

As for Dempster, he kept any objections to himself before voicing his opinion with fastballs. And if there was any doubt, he threw the first pitch behind A-Rod, followed up with two more inside, and, with the count 3-and-0, rifled a fastball directly at his left elbow.

So much for plausible deniability. The Fenway crowd roared with approval as Dempster zeroed in for the kill, and went wild when A-Rod got popped. After spending another day reviewing the video, even MLB czar of discipline Joe Garagiola Jr. had no choice but to announce that Dempster was punished “for intentionally throwing at and hitting” A-Rod.

That may be the letter of the law, but it doesn’t capture the bigger implications of the crime in this case. In asking around for a precedent, the most recent one we heard was Cole Hamels drilling Bryce Harper, then a 19-year-old rookie, square in the back last year as an “old school” welcome to the Show.

Unlike Dempster, Hamels admitted after the game that he hit Harper on purpose because he remembered when he was a rookie “the strike zone was really, really small and you didn’t say anything because that’s the way baseball is.”

Maybe Hamels’ views are a bit extremist in this day and age, but hey, at least he owned up to it.

But Dempster wanted it both ways: To be the tough guy to teach the big, bad PED cheat a lesson and yet still duck behind the lame excuse of having not one or two or three pitches get away from him — but four.

We can see why MLB looked at the Hamels incident. Both pitches were delivered with malicious intent. But with the unpopular A-Rod, this sort of treatment could become a trend, and a stronger penalty might help keep the team-sponsored vigilantes off his back. Bud Selig is no fan of A-Rod, and protecting someone he believes is Public Enemy No. 1 is not a huge priority for his administration.

Even so, there’s more at stake here, and A-Rod is not the only one at risk of getting hurt. What if he gets plunked on the leg, the benches empty, and Brett Gardner breaks his arm in the melee? Or CC Sabathia chooses to retaliate one night and fractures Evan Longoria’s wrist, altering the AL playoff races?

We get it. Everyone hates A-Rod, and his legal hit squad is making life miserable for anyone remotely connected to his suspension, including his own team. But first and foremost, MLB has an obligation to protect all of its players, and letting Dempster practically skate for his Gladiator act Sunday night is not a good enough deterrent — and leaves the door open for another Dirty Harry.

“That’s my concern,” Girardi said. “And we’ll find out. I sure hope not.”