In drenched, dreary Detroit tonight, on the final day of September, the Baltimore Orioles found out that they won't be playing any significant baseball in October this year. This soaked evening, as their game with Detroit was deluged out, the Birds were eliminated from a pennant race in which they never truly seemed to have a part.

The Orioles will make up tonight's rainout by playing the Tigers at 1:30 p.m. Thursday, but the game will be important only to the Tigers.

Tonight, on a night when the Orioles were licking their wounds after hearing that a 10-5 Milwaukee victory over Boston had erased their last playoff chance, the Birds were also opening new gashes in their tattered team psyche.

Quiet, respected 17-year veteran Mark Belanger, angry at his almost total inactivity for a month and upset that the Orioles obviously have no intention of signing him for '82, blasted Manager Earl Weaver. And Weaver fired back.

News that the Orioles were eliminated was routine. "We've been on a tightrope a long time," said Ken Singleton. "It's no surprise."

"We can still end up with the best record in the American League for the whole season and a lot of us want to do it; and I hope Cincinnati does the same in the National League," Ray Miller, the pitching coach, said bitterly. "Then the teams with the best record in both leagues will be left out of the playoffs.

"That will show how inexcusably assinine this whole split-season was."

While elimination had been feared and expected, the Belanger-Weaver episode was a shock.

"I can look back on five or six games in just the last month that Earl Weaver has helped us lose with his managing," said Belanger. "I haven't played in the late innings on defense in weeks and he's never told me why. Earl doesn't tell anybody anything . . . Weaver's trying to bury me."

"I thought I was doing Mark a service, after all these years, not to use him as a caddie in the late innings," Weaver rebuted. "I thought that's the way he wanted it. When people say to me: 'You'll never retire. You love it all too much,' I think of the tears that came to my eyes when Paul Blair said the things about me that he said at the end of his career, and now Mark, and a lot of others, too, over the years. In fact, almost all of them as they get older. That hurts. I'm not sure I want to be hurt anymore."

It's been a long month for Weaver and his flock. First, Weaver and pitcher Jim Palmer had a screaming match, followed by a week of mutual silence, followed by an extremely uneasy truce. Last Saturday night, after Weaver changed pitchers in the bottom of the ninth, Palmer said: "We messed up tonight. We forced the little man to think."

Next, Weaver was arrested and charged with drunk driving as he returned home from a Baltimore restaurant with his wife.

Next, Weaver's traditionally torrid September team has played stale 14-13 ball this month; many a Weaver decision has failed or backfired. Now, finally, Belanger -- a player Weaver has managed 14 years, including two in the minors -- says to anyone who asks: "Earl just isn't managing the way he used to . . . He used to treat me as though I had the best glove. Now, all of a sudden, I'm nothing. There are only two explanations for him not using me in the late innings of games. Either I'm hurt, or I can't play (adequately) anymore. Neither one is true. I sure don't think I'll be playing in Baltimore next year, but I definitely want to play somewhere."

As for criticism that he has had a bad managing year, Weaver says sardonically: "I've lost 45 games for us this year, because that's how many times I wrote out a losing lineup. And I've won 56 games, because I wrote those out, too . . . We've got the best record in the league in one-run games (20-7). If we win three of our last four games, we can still end the season with the best record in the league for the whole year. That's worth second-place money."

To Weaver, occurrences such as Belanger's blast seem to happen with a sad annual regularity that is all the more depressing for its predictability. For a decade, one of Weaver's controversial characteristics has been his invariable policy of sticking with slumping veterans far longer than most managers would, hoping for one last return to form. The list includes Blair, Brooks Robinson, Boog Powell, Don Buford, Mike Cuellar, Lee May, Pat Kelly, Palmer (this year) and, for the last three years, Belanger.

Weaver's famous line -- "I gave Mike Cuellar more chances than my first wife" -- might well apply to every player who has ever contributed heavily to a Weaver team.

"It just makes me sad to see Mark go out this way," said Weaver with a shrug. "It sure ain't the first time. Every player's got a choice (as he ages): he can blame his body or he can blame his manager."

In Weaver's experience, how many players have reacted to his finally removing them from the lineup at the end of their careers by retaliating against him publically?

"Maybe 80 percent," he said. "No, I guess it's more like 90. The one who comes to mind that didn't was Brooksie (Robinson), and he had as much right as any of 'em to get mad. I understand it. I guess I kind of expect it.

"But I hate it. It hurts me more than anything in this job."

Then, Weaver put on his public face, strolled into his clubhouse and sarcastically said, loud enough for players to hear: "Like (Baltimore trainer) Eddie Weidner used to say, 'Hold on fellas; there's only four more days left (in the season) and then we can pick our own friends."