CLEVELAND — Once upon a time, in a big postseason game, a major-league manager brought his best pitcher back to start, on the road, with just three days’ rest.

The pitcher shut out the opposition, cementing his reputation as a big-game pitcher, the manager looked like a genius, and the Florida Marlins lived happily ever after.

That pitcher, of course, was Josh Beckett. The start was Game Six of the 2003 World Series, just four short years ago.

But it might as well have been 40 years ago. Those days are gone and like it or not, they’re not coming back.

The manager of the Marlins was Jack McKeon, who was 67 years old at the time, a baseball man from another time, making one last decision. McKeon didn’t care much for pitch counts or baselines that highlight shoulder strength.

He had a big game to win, so he went with best pitcher. End of story.

It’s not that simple anymore, and we can debate until spring training whether that’s good or bad. It doesn’t change the fact that when you sit down and watch Game Four of the American League Championship Series tonight, you’ll be watching Tim Wakefield and not Beckett.

The Red Sox said as much Sunday. In case anyone missed it, they said it again yesterday afternoon. And for dramatic effect, they said it one more time last night, minutes after they had dropped Game Three, 4-2 to the Cleveland Indians.

Josh Beckett is not walking through that door — at least not the one that leads to the mound at Jacobs Field.

If that strikes some as shortsighted, or overly dogmatic, it won’t change the Red Sox’ decision. Baseball 2007 isn’t about gut feelings or instincts or emotional reactions.

It’s about protecting investments and managing resources and long-term plans.

The case for Beckett tonight — and, should he be needed, Game Seven on Sunday, too — is a simple one. By pitching him in Game Four on three days’ rest, the Sox would get three starts from their best pitcher and none from their fourth-best (Wakefield).

Sounds simple, right?

Except it isn’t. By bringing Beckett back tonight, the Sox would also be asking for Curt Schilling to pitch on regular rest in Game Five on Thursday night and Daisuke Matsuzaka to go on regular rest for Game Six on Saturday.

Neither of those moves sounds like a good idea now. Schilling’s shoulder, even after a prescribed seven-week layoff in the middle of the season, is hardly at full strength. When he had 12 days rest for Game Three of the ALDS against the Los Angeles Angels of Anaheim, his command was precise and his fastball had a little extra life.

Pitted against a far more powerful Cleveland lineup in Game Two on Saturday, Schilling wasn’t quite as fine and his fastball wasn’t quite as lively. Hours after his removal from the game, Schilling was still mystified as to how Jhonny Peralta had driven what seemed like a near-perfect pitch out of the ballpark.

As for Matsuzaka, he worked last night with the benefit of nine full days rest — and still couldn’t get out of the fifth inning. He labored, he nibbled and he seemingly went to a full count on every other Cleveland hitter in the lineup. Asking him to pitch on regular rest would be folly.

That said, the current plan, the one the Red Sox are so steadfast in their support, is something of a crapshoot, too.

Wakefield hasn’t pitched since last month. Schilling could benefit more by pitching in Jacobs Field, rather than the less-forgiving Fenway Park. And then there’s the matter of Matsuzaka pitching Game Seven with the pennant on the line.

But first, the Red Sox have to get there. And if they get there, they will take their own route, the one not wedded to emotion or instinct or gut reactions.

They’ve planned this thing out to within an inch of its life, studied their options and cast aside any notion of risking their $40-million investment.

You don’t have to like it. Chances are you don’t. But that won’t change their minds or the identity of tonight’s starter.

Bring back Josh Beckett on three days’ rest? Those days are gone and they’re not coming back.

smcadam@projo.com