THE REAL EXCITEMENT was supposed to have started tonight with the American League championship beginning in Kansas City. But you could have fooled 34,000 jumping afternoon, along with millions of television watchers like ourselves - all of whom saw the Yankees and Red Sox playing the game of baseball as its inventors dreamed it would be played. It was a sudden-death playoff, and sudden death was that Yankee shortstop Bucky Dent seemed to bring the Red Sox with a three-run homer over the "green monster" left-field wall in the seventh, making the score 3 to 2. When the yanks scored again, and again in the eighth, with Reggie Jackson hitting a solo job into the center-field seats, you could almost hear the Bostonians start to think about hockey.

But the Sox were still playing their own game. They had not choked, as everyone thought they would, having once led the Yankees by 14 games. When the Yanks overtook the Sox a couple of weeks ago, the Sox were supposed to lie down and whimper. Instead, they started winning again, earning the tie that brought them to yesterday's confrontation.

That their final comeback fell short was incidental to everyone but diehard Red Sox and Yankee fans - who admittedly account for much of the population. For the rest who simply cared to see a game played to the hilt, the effort of the Sox was a triumph. In the eigth Jerry Remy doubled; Carl Yastrzemski singled; Carlton Fisk and Fred Lynn followed with singles, and the score was 5 to 4. Goose Gossage, the Yank reliever, began to look like a big taunted kid. Yet he struck out George Scott, and the teams were in the ninth.

By then the shadows that had hovered behind the plate in the early afternoon covered half the outfield. The crowds, which had started the day kind of quiet, was in a constant yell, as the Yankees went down in the top of the inning. Dwight Evans, a pinch hitter for the Sox, flied out to left. Rick Burleson walked-the tying run was on. Remy singled - the winning run was on. Jim Rice flied out to right. Yastrzemski was up. The Yankee coaches huddled in the dugout.

Of course it had to be Yastrzemski. The Red Sox warrior-hero of so many years, ancient now at 39, had already hit a home run and had driven in two. Who else could end such a game? When he popped up, all Boston gasped, and everybody knew the season was over. Some may even have thought the Red Sox had lost. But nobody lost.