For perhaps the first time since he joined the Cosmos in 1976, soccer's most prolific goal scorer, Giorgio Chinaglia, is expected to play a secondary role to a teammate when the NASL-champion Cosmos visit the Washington Diplomats at 8 o'clock tonight at RFK Stadium.

Chinaglia, the NASL's all-time leading scorer, has scored a league-high 17 goals this season, but has scored only one in the last four games, that resulting from a penalty kick.

So Washington will focus its defense on a 21-year-old terror named Roberto Cabanas.

The young Paraguayan has scored 11 goals in 14 games, including four against Dallas May 30 and two against Toronto Sunday. The Cosmos wanted Cabanas so badly that they agreed to let him return home during the season to play for Paraguay's national team.

"Cabanas has become a difficult problem now," Diplomat Coach Ken Furphy said yesterday. "He is now the most dangerous of their forwards."

Furphy will probably assign his best defender, Peter Carr, to mark Cabanas. And 36-year-old Eddie Colquhoun, the assistant coach who started his first game of the season Sunday, probably will defense Chinaglia.

Colquhoun, at 6 feet, 175 pounds, is almost as big as Chinaglia, who likes to use his powerful body to back a defender into his own goal. The smallish Carr held Chinaglia scoreless in the teams' first two meetings this season.

Carr and Chinaglia, who apparently dislike each other, will appear on a morning talk show today. Chinaglia declined initially, but the owners of the Cosmos asked him to appear after Diplomat owner Jimmy Hill complained.

Carr suffered a gash in his forehead in Sunday's game and has a slightly bruised leg and strained knee. But the tough center back will start.

"We talked to the Toronto coaching staff and they said Cabanas is very, very dangerous," Furphy said.

On May 23, in the Dips' 3-2 victory at RFK, Cabanas scored the Cosmos' second goal and almost tied the game near its end with a blast off the crossbar. Cabanas' only apparant weakness is that he can be bullied. And Carr, nicknamed "Biff," could be rough with the youngster.

The feud between these teams began simmering again when striker Paul Cannell, a part-time broadcaster with WDVM-TV-9, accused the league's referees on a Monday news show of favoring the Cosmos.

"When you play the Cosmos, you also have to play the referees," said Cannell, who has had several memorable run-ins with the Cosmos. "Not only are the NASL officials the worst in the world, but they're probably being paid by Warner Communications (the parent corporation of the Cosmos).

"It's like a heavyweight champion. You have to beat him into the ground before the officials will decide to go 50-50," Cannell said yesterday afternoon. "One time, in the old days, Pele gave an opposing guy a yellow card."

The Dips are brimming with confidence after their 2-1 victory over Chicago, the team many believe is as strong as the Cosmos.

"They (the Cosmos) have probably got a little fear in them now because they know they're going to be in a third battle tonight," said Duncan Hill, Dip general manager. The Dips trail the Cosmos by 40 points in the race for the Eastern Division title.

The Diplomats signed goalkeeper Jim Messemer, their 6-foot-5, 200-pound second-round draft choice who graduated last month from Texas Tech. The Dips had been carrying only one keeper, Jim Brown, since backup Dale Baxter was injured in an automobile accident. Baxter, who went home to Toronto to recuperate, is expected back for tonight's game.