The Washington Diplomats, in their first "must" game of the North American Soccer League season, host Eastern Division leader Fort Lauderdale tonight at 8 in RFK Stadium.

The Dips (5-4, 42 points) missed an excellent opportunity to move up in the standings last week, losing at Rochester, 2-1. But the Cosmos (5-5, 48) and Fort Lauderdale (6-3, 50) have kept the division tight by losing regularly in recent weeks.The Strikers started strong, winning their first four including a 1-0 decision over the Dips. But inconsistency has plagued coach Ron Newman's club and it has dropped three of its last five games. The Cosmos have lost two straight.

Tampa Bay leads the division with a 7-3 mark and 63 points.

"It's a must game for us too," said Newman. "Washington has been creeping up on everyone the past few weeks and could move past us if we don't play well."

Before Washington's ill-timed loss to a bruising, physical Rochester team, it had begun to settle down after a horrible start. A slew of English newcomers had become adjusted and the Dips racked up four straight victories.

Coach Dennis Viollet was upset at Rochester's tactics last week. "It was more of a battle than a soccer game," he said.

Viollet will be without the services of defender Ken Hill and forward Alan Green, both nursing bruised leg muscles. Winger Jimmy Redfern returns to the starting lineup as will Gary Darrell. Both have missed five games because of injuries.

"This game is a big step toward a successful season," said Viollet. It's an important game for us but it's not the end of the war. Every tomorrow's game is important to me.

The Fort Lauderdale early-season victory on a goal scored with only 26 seconds gone in the game, still gives the Washington players nightmares.

"It put a lot of pressure on us, that's for sure. A team scores that fast," sai Mike Lester, a forward. "We have to win this one. You want to beat anyone in your group. I wasn't too impressed with them the first time and we should have beaten them."

If Washington is to make a serious run at the title, it must get more offense.

Viollet has used a different set of front line players on at least six occasions and the team has produced only 12 goals (14th in the league).

Even Lester admits the offense has not been up to par.

"We played well together a couple of games. In other games we sort of faded out," said the England native who has one goal this year. "The last two games were on rather small fields and we got a little crowded. On our field, we have more room and we can move bout."

This is the first of six Saturday night games at the stadium. The first 8,000 people will receive a NASL poster. It is also Boy Scout night.

Abraham Klein of Israel, one of three new international referees hired by the League in an effort to upgrade the officiating, will make his first appearance in the United State tonight. Klein officiated in the 1968 and '76 Olympics.