Why him? That's what they all said yesterday when they heard Orioles pitcher Mike Flanagan ruptured his Achilles' tendon in a charity basketball game Wednesday night.

Why should another total fluke injury befall one of the toughest, funniest, kindest, most competitive and best-liked players in baseball?

"Why does this stuff keep happening to such a nice guy?" wondered the Baltimore Orioles' new coach, Terry Crowley, who saw a player for a faculty and alumni team from Laurel High School step on Flanagan's ankle in a meaningless away-from-the-ball misstep that sent the left-hander into career-threatening surgery yesterday. Flanagan will be in a cast two to three months and could be out for six months, at the very best.

"No," amended Crowley. "Not a nice guy. The best."

"I can't think of anyone who has ever had worse luck," said the Orioles' general manager, Hank Peters, who has been in baseball 38 years. "I talked to Mike this morning before his operation and I told him, 'God has smiled on you many times, but, boy, he sure has kicked you in the butt, too.' "

Flanagan, 33, who routinely ignores being hit with line drives and who has started more games and pitched more innings than any other pitcher in the American League the last eight years, had only two tight-lipped comments to his teammates after limping from the court:

"It's not good," and, "Something's wrong down there."

Normally, he is one of the game's quickest wits and genuinely wry phrase makers. But he's learned not to joke about his body. In 1983, he twisted to field a routine grounder and was disabled 80 days with a partially torn knee ligament. Doctors assumed he'd be out for the year, yet he made it back for the pennant race with a huge knee brace he used through the '83 World Series, then into the '84 season.

So great was the professional respect for him that few opponents bunted after his return, though most runners could beat him to first base. It wasn't a gentlemen's agreement, since there was no quid pro quo; it was just deference toward a pitcher whose personal code never let him throw a brushback, use an illegal pitch or even "pitch around" great hitters whom he made it a point of honor to challenge, because "that's what the game is about to me."

When he took the brace off, ahead of doctor's orders, he nearly regained the form that won him the Cy Young Award in 1979, yet his team hit so poorly for him that, despite a top-20 ERA, his record was 13-13.

He didn't miss a start from '80 through '82, yet spent two winters rehabilitating a shoulder that specialist Dr. Arthur Pappas said was "the closest thing to a torn (rotator) cuff that I've ever seen without being one."

Off the field, Flanagan's luck has been just as spectacularly mixed. His wife Kathy, who was his hometown sweetheart, had two miscarriages and was told she could never bear children. This began three years of depression for the Flanagans, who refused medical advice that would have ended their last chance for children. Flanagan said, "They'll find something." And they found something. In July 1982, Kathy Flanagan gave birth to the first in vitro fertilization baby born in America through normal delivery.

"That was kind of a miracle," Peters recalled yesterday. "I told Mike that he'd had some real good breaks, too."

Despite that, furrowed brows were the order of the day yesterday. No one associated with the Orioles could remember a player who recovered fully from this very rare injury. Such a rupture ended Bill White's career, and Bobby Tolan was never the same.

"Oh, no," said the pitching coach, Ray Miller, over the phone from New Athens, Ohio, after the news arrived. "Been saying something like that was going to happen . . . I guess there'll be a stink now about them playing basketball in the offseason."

"We've had this basketball team for 15 or 16 years," Peters said, "and no one has ever gotten hurt that we can remember. I wish we could put 'em in a glass cage between seasons, but these guys are athletes, period, and they're going to do something physical. At least this way, they are supervised. We had two trainers there when it happened."

Although Flanagan was playing in his first basketball game this season, Crowley said, "He was in the best (offseason) shape I've ever seen him in. He said he was swimming an hour a day and running. We told him to stay out of the action and he was. He'd only been in the game a few minutes. Eight of the other nine guys on the court had run past him and the last guy tripped over his ankle. You'd think this would happen against a clumsy team or a dirty team, but they were very good and very clean. In fact, we thought we were going to play against faculty members, but it ended up being the alumni from a state championship team two years ago. Eight leapers."

Far from being a basketball klutz, Flanagan still holds the New Hampshire state record for points in a state championship tournament game (44) and was picked to the state's five-man all-decade team for the '70s.

The Orioles spent yesterday introducing their newest pitcher Don Aase, to the media. He came back last summer after 23 months of rehabilitation. He'd had a tendon in his left wrist grafted into his right (pitching) elbow.

"I'd say one word to Flanagan: patience," said Aase, whose career was in as much jeopardy then as Flanagan's is now. "The experience is so frustrating and depressing. You go forward, then back, forward, then back, and each time the forward is a little further. The worst thing is to try to come back too soon. I had doctors who sat on me."

Flanagan, who has a $600,000-a-year contract that's guaranteed through '86, always has been the height of impatience, always wanting to help the team too much and often pitching while limping or compensating for some injury.

One question haunted the Orioles above all others yesterday: Will this end his career as a top pitcher?

"Maybe anyone other than Mike, you might think so," Peters said. "I think he'll come back." Maybe he meant Flanagan is a quick healer and hard worker who overcomes injuries. Or maybe he meant that, of all the players in baseball, none deserves one good break more than Mike Flanagan.