Jordan Zimmermann has a change-up. That sounds innocent enough. But in baseball, that’s like saying a gorilla just learned how to use a machine gun.

Every pitcher who lacks a change-up is dying to get one. It makes every other pitch look better by contrast. But there may not be a hard-throwing hurler in the whole sport of whom it has been said more often, “How good would he be with a change-up?”

Now, it looks like we’re about to find out. On Monday, the normally stoic Zimmermann looked giddy after he had retired 18 Tigers in a row and baffled many of them with his new change-up.

“I had all four pitches working,” the Washington Nationals’ right-hander said. “It’s hard to come by those days. They don’t come by often. You feel like you can’t do anything wrong. I threw a lot of good change-ups that were falling below the zone.”

Some of us have never seen him smile, much less grin, look sheepish, make quips and then disappear saying, “Let me sleep on this.”

The Post Sports Live crew discusses whether Stephen Strasburg will pitch over 100 pitches in at least one-third of his starts this season. In his first year back after Tommy John surgery, he topped 100 pitches in only five of his 28 starts. (Post Sports Live/The Washington Post)

Sweet dreams, indeed.

“The change is that equalizer pitch,” shortstop Ian Desmond said after watching Zimmermann hang a meek 0 for 12 on Prince Fielder, Miguel Cabrera and Victor Martinez — three of the best sluggers in baseball — as well as Detroit standouts Torii Hunter, Omar Infante and Alex Avila.

Until that game, Zimmermann might have been the Nats’ biggest worry. Ten days earlier, he gave up eight runs against his Cardinals nemeses and said he had a “dead arm.” But he predicted he would bounce right back, which he did with a solid outing. But few were prepared for Monday.

Zimmermann has talked about his new change-up all spring. But his pitching buddies have teased him that it’s just “a February change-up,” that will disappear in the regular season as soon as somebody lights one up with a 450-foot charge. The new pitch likely won’t go back in the ball bag now.

The Tigers were so off-stride that their groundouts reached fielders after many weak dribbles. Zimmermann needed just 67 pitches for six innings and had to go to the bullpen to throw 20 more just to make it a decent day’s work.

Even without an effective off-speed pitch that looks like a fastball when it leaves his hand, Zimmermann has had an ERA+ of 128 the last two seasons. That means his ERA (2.94 last year), adjusted for his home park, was 28 percent better than the league norm. How good is that? Just 20 starting pitchers since 1920 have an ERA+ of 128 or better. That’s why the change-up matters. From such a starting point, how much better can he get?

Still just 26, Zimmermann is just entering his prime. How good does he actually want to be? His combination of elite ERA but a mere 20-19 record the past two years makes you wonder if he has simply lacked run support or whether something in him shrinks from conspicuous success.

Zimmermann prefers to be overlooked. He deflects attention without being hostile. His excellence earns opponents’ respect, but he’s not splashy enough to draw a crowd. He loves it this way, standing just a bit to the side, observing, making a wry remark to a teammate, playing third banana behind Stephen Strasburg and Gio Gonzalez on the Nats’ top-ranked staff.

He’s Wisconsin winter icy on the hill, never even changing expression, always attacking, sometimes throwing too many strikes because his personality demands competitive aggression. Deception, like a change-up, has seemed almost like an affront to his style.

Something may have changed in Zimmermann last October. In his first start on a big playoff stage, he was shelled in St. Louis.

“The Cardinals own me, pretty much. And I own the Tigers, it seems like,” he said this week.

However, in Game 4, on two days’ rest, Zimmermann struck out the side in the seventh inning in relief, touching 97 mph. For the first time in his career, he showed emotion on the mound, sucking the huge crowd into the game with his 10-pitch, nine-strike domination. As he walked off, he pumped his fist and screamed. The crowd that he’d incited to its feet, continued to stand, for the next hour, until Jayson Werth’s homer.

Who was that guy, Zimmermann was asked.

“He only comes out on special occasions,” he replied.

One key for the Nats is whether Zimmermann remains an 8-11, 12-8 pitcher that purists admire, who racks up quality starts, fields his position deftly and is a good hitter, but who hits a wall at times because he lacks a shift of gears, an alternate style of attack. Is that about to “change” or will Zimmermann revert to his hard-and-harder comfort zone in real games?

After all, when you’re one of the 10 hardest-throwing starters in the game, it feels unnatural to get in a tight spot and respond with a first-pitch change-up to get an anxious fastball-hungry hitter to roll over his hands too quickly and ground out. When you’re behind in the count 1-0, 2-0 or 2-1 to a great slugger, it’s tricky to grasp that “throw slower” might be better. And for Zimmermann, who has trouble finishing off hitters because they foul off so many of his pitches, the change-up might be his “expand the zone” pitch. It takes about 10 of ’em a game, in the right spots, to confuse your foes.

The Nats assume Zimmermann is still maturing with the change-up just the next logical step, like Ross Detwiler trusting his curve more or Strasburg working on a later-breaking slider. “The change is a big pitch for him,” Johnson said.

“I think he’s an underrated pitcher because he has flashy stuff, but not a flashy motion,” Drew Storen said. “He throws a heavy hard 95, but it seems even faster to hitters. His delivery is compact. So, the ball jumps on ’em.

“This change-up gives him something in that middle gear between his fastball and curve that isn’t a slider. It’s going to be fun to watch.”

Trickery comes about as naturally to Zimmermann as doing stand-up comedy. But baseball almost requires it. If he faces the Cards again some October, will he be inclined to subtlety? Will he trust a touch pitch or just go back to the comfort of the Good Gas?

Zimmermann’s “February change-up” made it to March. Come on, April.