NEW YORK – After Mariano Rivera got routed in his major league debut, manager Buck Showalter offered an observation.

“Hopefully, Mariano will learn from it,” Showalter said.

More than a decade later, he has a World Series MVP, four championship rings and now 400 saves under his belt. Pretty good for a guy who throws only one pitch.

“The best ever, no doubt it,” Dennis Eckersley said Monday.

High praise from the lone reliever to make it into the Hall of Fame on his first try.

“No comparison!” he said. “I didn’t have the stuff he did. Not at all. I’ve been blowing his horn for years. He’s electric.”

Put it this way: Put the New York Yankees’ ace closer on the Atlanta Braves from the start of his career, and they’re probably a dynasty rather than a postseason disappointment. Instead, Chipper Jones was reduced to chuckling in the dugout as he watched Rivera’s cut fastball break three of Ryan Klesko’s bats while finishing off a 1999 Series sweep against Atlanta.

“He has the single best pitch ever in the game,” Chicago White Sox slugger Jim Thome said.

“I mean, those left-handed hitters know what’s coming and he still eats them up. How does he do that?” Eckersley said.

And while Metallica’s “Enter Sandman” is Rivera’s signature song, he has another trademark sound at Yankee Stadium: the cracking of Louisville Sluggers.

“Every time I go out there, it’s business,” Rivera said.

The 36-year-old Rivera has done it enough since becoming a full-time reliever in 1996 that his place at the Hall of Fame is set.

Rivera is the very definition of a Cooperstown closer.

“You don’t even have to look up his numbers,” said Hall voter Paul Hoynes of The Plain Dealer in Cleveland. “You don’t need to go to ‘The Baseball Encyclopedia.’ We’ve all seen what he’s done.”

Of course, Rivera is not automatic. While he’s posted a record 34 saves in the postseason, he’s blown three memorable chances: the Boston Red Sox got him in Game 4 of the 2004 ALCS. Arizona’s Luis Gonzalez nicked him in the ninth inning of Game 7 in the 2001 World Series. And Sandy Alomar Jr. tagged him in the 1997 playoffs.

Rivera made his big league debut in May 1995 as a spot starter for injured Jimmy Key. The California Angels tagged him for five runs in 3 1-3 innings of a 10-0 loss.

Rivera bounced around the rotation the rest of the year, and Showalter opined that minor league stats don’t necessarily translate into major league success. Yet following an impressive outing, Rivera showed that he believed in himself.

“There is no doubt in my mind,” he said. “I know I can pitch here.”