The Washington Diplomats, 2-1 losers to the defending North American Soccer League champion Cosmos Wednesday night in RFK Stadium, are still seething.

But not from the defeat.

The Dips' front office is incensed because the Cosmos, who already had wrapped up the Eastern Division title and the home field advantage for the first round of the playoffs next week elected to rest nine of their 11 starters for the Washington game.

After a protest call from the Diplomats' president, Steve Danransky, to the Cosmos' president, Ahmet Ertegun, and the league office, the Cosmos relented and flew in two regulars, Steve Hunt and Vladislav Bogicevic, in time for the game.

"That didn't help us," said Danzansky, who has filed a protest with the league office. "By that time, the damage had been done. We know there's nothing in the rules stating a team must play its regulars, but in the interest of soccer and fair play, no team should be allowed to put a team on the field with no first-class players. Especially since the game was so important to us."

Aside from the fact the Dips are locked in a struggle for a playoff berth, the absence of such world-famous players as Franz Beckenbauer, Carlos Alberto and Dennis Tueart probably cost Washington its biggest gate of the year.

Only 17,045 (revised from the announced crowd of 16,207) showed up for the game. A crowd of 20,000 to 15,000 was expected.

"We had an advance sale of 17,000. Some of them were no-shows because of the publicity," said the Dips' general manager, John Carbray. "We lost at least another 5,000 walkup. As it turned out, the Cosmos put a pretty good team on the field, but nevertheless that move hurt us."

Several of the NASL teams, especially those that have not drawn well, make almost as much money from a Cosmos visit to their city as they do for the rest of the season.When The Cosmos signed the incomparable Pele to lend some credibility and give exposure to the then-fledging league four years ago, they immediately were given the burden of supporting the league.

"Certain teams are better draws, of course," said Danzansky. "And teams build up the games. We were looking forward to it just like Abe Pollin and the Bullets would look forward to the 76ers coming to town. Same principle."

Cosmos Coach Eddie Firmani defended his decision to leave his stars home.

"I have an obligation to my players," he said, "and some of them are hurt. It's a fine thing when you can't pick your own bloody lineup. My moral obligation is to my club."

Firmani and Ertegun insisted the Cosmos starters had been held out because of injuries. But after a call from Ted Howard, the NASL director of operations, The Cosmos sent out a hurried call to Hunt and Bogicevic.

"We can't force anyone to play players," said Howard. "There's nothing in our league rules that say that, I just suggested (that) under the circumstances maybe The Cosmos should reconsider their decision. Teams are working to earn playoff berths and a decision like that could affect quite a few teams."

Ertegun said it was more important to ready his team "for the playoffs and the exhibition world tour after the season."

Danzansky said the Diplomats did not promote The Cosmos game as much as in past five years. Last year, 31,283 came to see Pele in his final game at RFK. In that contest, won by Washington, 2-1, The Cosmos held out several starters because of injuries.