YOU never completely understand how good you had it until you don’t have it anymore. Take 2000, when the Yankees breezed to 15 losses in their last 18 regular-season games, dropped Game 1 in Oakland, and Joe Torre didn’t fully realize just what good shape he was in because he had four games remaining and Mariano Rivera to pitch in all of them.

Or take 1998, when the best season ever had by the best franchise there ever was teetered on the brink, down 2-1 in Cleveland, without a bead of sweat appearing a single pinstriped brow. Talk about a set-up. There was only one Rivera, and the Yankees had him.

Or how about 2001, when the sky figured to finally fall because of the engineering flaw that had sent Jeff Nelson to set up games for the Yankees’ ALCS opponent, and all that Nervousness over Nellie proved misplaced because Rivera still was in place.

Between the time Sandy Alomar reached out in the eighth in 1997 and Luis Gonzalez blooped one in 2002 it hadn’t occurred to the Yankees that they could lose any postseason game other than ones Kenny Rogers pitched. Twenty-three consecutive successful Rivera saves fell between two events as incomprehensible as the end of the firmament and the drying of the oceans, four World Series titles into the books by a manager strictly going by a book it now takes a stiff upper lip not to read and weep.

This October, Torre almost certainly won’t have Rivera every game and may not have him at all. And it won’t necessarily take Steve Karsay being rotten for the effects of being so spoiled by Rivera to show up.

If Karsay and Mike Stanton are being saved for the ninth, who pitches the seventh, assuming Ramiro Mendoza is used in the eighth instead of in the sixth? What game does a tiring, fortyish Yankee starter have to be stretched if Jeff Weaver had to relieve the previous night to start the fifth?

And what Hall of Fame manager-to-be now has to go to work for a living, carrying a committee on his head into the ninth.

So far, it’s light as a feather. Last night, Karsay pitched a scoreless eighth and ninth to pick up his fifth save in five opportunities since Rivera’s shoulder went bad again, saving Andy Pettitte’s ninth victory. Karsay, Stanton and Mendoza are 9-for-9 in saves since Rivera went down.

But next month the Yankees will be playing an October team, not May wonders like the Red Sox, which as Torre’s team sails into a great unknown, could bring tears to eyes in remembrance of those eight-inning lockups of yore.

“We’re trying to not put it all on one guy,” said Torre. “It’s easier to compare a Rivera to three than one because I don’t think one can hold up under that.

“I would like to let them know their role. But their role is to be ready when the phone rings. If it’s for Mendoza in the sixth that night, then Karsay is the closer and Stanton the guy to get a left-hander out in between. Before, bringing in Mo was the extent of it.

“It’s a lot more pressure on me. But we haven’t to this point given away anything. The difference in our club this year from last year, we left spring training with a much deeper bullpen.

“I feel better about our starting pitching than a month ago. I don’t have anxiety right now. Believe me, nothing was as stressful as going into the playoffs losing 15 out of 18. It got so bad in Baltimore when we had a chance to clinch I didn’t even get a laugh when I said, ‘Let’s drink the champagne before the game.’ “

Because they had Rivera, the Yankees didn’t get the joke.