PHOENIX – This was exactly how it was supposed to work out: Orel Hershiser joins the Mets, fills a spot in the back of the rotation, hangs in there for six innings at a time, lends a mature touch in the clubhouse, reaches double figures in wins, helps the Mets reach the postseason for the first time since he knocked them out of the playoffs in 1988.

The Mets haven’t reached the postseason yet, but Hershiser has done everything else expected of him and history suggests the Mets haven’t yet seen the best he has to offer.

Hershiser was perfectly suited for this Mets team. He throws groundballs and no infield catches them better than the Mets. He’s strictly a six-inning pitcher and few bullpens are better than the Mets’, which stands to get deeper and better when John Franco returns to action in early September.

The start Hershiser gave the Mets last night was pretty typical. He lasted 52/3 innings, allowed five hits, walked one and struck out five. He was charged with three runs. That figure would have been two if not for a fluke hit that didn’t clear the infield grass and squibbed off the broken bat of Steve Finley against Dennis Cook.

Historically, when the dog days of August are played out, they give way to the Bulldog Days of Hershiser.

The higher the stakes, the better Hershiser pitches. It’s been that way his entire career.

His career ERA in September and regular-season October games is 2.93, the best of any month by a good measure.

In 1262/3 postseason innings, Hershiser has an 8-3 record and 2.70 ERA.

“You go from a marathon mode to a sprint mode,” Hershiser said. “I definitely take on a sprint mode at the end of the season. Everything that happens in the year becomes background for what to do. How you’ve gotten hitters out during the season sets them up for how you go after them now. It’s time to break out some new things to surprise people.”

In ’88, Hershiser took to throwing sidearm breaking pitches in October, not that he needed to change anything from what was one of the greatest seasons in recent memory.

It’s not even his experience that makes him a valuable asset in a pennant race. It’s his personality. He’s a ham who enjoys the camera’s eye, loves the big-time atmosphere, will relish another appearance on the national stage.

Kenny Rogers is closer to his prime than Hershiser is to his, but that doesn’t mean he would be a better pick for the post-season rotation.

The bullpen will be even deeper in the postseason, when the Mets will drop to a four-man rotation, which will make Hershiser’s penchant for lasting only six innings less of a factor. Al Leiter, Rick Reed, Hershiser and either Rogers or rookie Octavio Dotel, whichever pitcher has a better September, makes the most sense for a post-season rotation.

Hershiser (12-10, 4.68) won’t be the story in the postseason, should the Mets get that far, the way he was in ’88. He seldom is the story anymore because at this stage of his career he needs the help of the bullpen behind him. The story with the 1999 Mets has been one of late-inning comebacks from an offense so loaded the first seven batters in last night’s lineup were hitting .300 and a bullpen that makes those comebacks stand up.

The Mets had neither staple going for them last night at Bank One Ballpark, where the Diamondbacks came from behind for a 5-3 victory to even the series that concludes tonight.

Hershiser got off to a great start last night, going through the order the first time without allowing a runner to reach base. His biggest mistake of the night came when he walked Tony Womack leading off the fourth. Predictably, Womack turned that walk into a virtual double by stealing second base for his National League-leading 59th stolen base.

Womack scored on Jay Bell’s 89th RBI to tie the game.

Erubiel Durazo’s run-scoring single with two outs in the sixth, the third base hit of the inning allowed by Hershiser, trimmed the Mets’ lead to 3-2 and convinced Mets manager Bobby Valentine it was time to go to his bullpen.

That’s the way it is now for Hershiser, who turns 41 on Sept. 16. It wouldn’t be a winning formula for many teams. On most nights, it is for the Mets. Smart acquisition.

“I’m very satisfied with how the year has gone and I’m excited to be with this club,” Hershiser said. “It’s been really exciting for me.”

It’s only going to get more exciting, which means there is a good chance Hershiser is about to get better.