Riding the crest of the greatest triumph in the seven-year history of the franchise, the Washington Diplomats insist there is no chance of a letdown during tonight's 8 p.m. battle against the New England Tea Men at RFK Stadium.

"That one victory over the Cosmos the Dips won, 2-1, in a shootout last Sunday night at the Meadowlands gave us confidence in ourselves and one another," said Washington defender Bobby Iarusci. "There is no way we can let down now against anyone."

The victory assured the Dips of second place in the National Conference East Division and a playoff berth. Now 15-15 with 141 points, Washington would love to defeat both New England and Minnesota Sunday at RFK to finish better than .500 and gain momentum for the NASL playoffs.

"It was definitely a big win for us and there's no letdown coming now," said Dip coach Gordon Bradley. But we still have a long way to go in this season and I'm sure our attitudes are not going to change."

Led by 38-year-old goalkeeper Kevin (the Cat) Keelan and striker Bob Newton (14 goals, three assists), New England has won 12 of its last 15 games, including a 1-0 shutout over Washington July 12.

"They beat us but they didn't play that much better than we did that day," recalled Iarusci. "We were playing our last game of a four-game road trip and we had out more regulars than we had all year for any game. Now we are just about at full strength and we're at home playing on a bigger field."

Like Washington, New England has had its problems on the road. On their recent road trip, the Tea Men were thumped in San Diego, 4-0, and in Toronto, 3-0, before regrouping to beat California, 4-1. The visitors' defense has not played badly, having allowed 52 goals, six fewer than Washington.

The offense, which has had problems at times, may get additional life tonight if talented winger Keita returns after missing nearly half the season with a leg injury.

Washington will not have the services of midfielders Juan Jose Lozano, whose goals sent the Cosmos game into overtime, and Thomas Rongen. Lozano reinjured his calf muscle and while X-rays reveal no break, he was deactivted for the rest of the season. Rongen has a severly sprained ankle and will not play tonight.

Another major concern for Bradley is the status of Johan Cruyff. The Dips' field general, who has been spectacular the last few weeks, was knocked down by Cosmos goalie Hubert Birkenmeier during the shootout and s suffered numerous bruises. According to a Washington spokesman, Cruyff is "sore all over." He did not practice yesterday, but his status is probable for tonight.

Washington and New England (formerly Boston) have split 10 lifetime meetings. The Dips had won six of the last seven before losing earlier this year. . . . Though Washington's Alan Green was blanked Sunday, he is still fourth in the league in scoring with 54 points (23 goals, 8 assists) . . . Joe Horvath, out the last few weeks with a leg injury, has returned to the active roster in Lozano's place.