The Washington Diplomats, still trying to patch up their defense, meet the defending North American Soccer League champion Toronto Metros at 2:30 p.m. today at RFK Stadium in their season opener.

The defense has been a matter of concern for coach Dennis Viollet since the midfield and defense sagged badly during last season's stretch drive. The club finished with a 14-10 record and earned a playoff berth for the first time in its three-year history, but it was clear that an off-season talent hunt was in order.

Viollet signed six British players and acquired several others in trades and through the college draft. Two of the Englishmen - Alan Green and Graham Oakley - are still playing for Coventry.

Today's starting lineup probably will include three of the Britons at defense - Don McAllister, Ken Hill and Mike Lester. Lester is listed as a forward but is considered the most versatile Diplomat.

Yesterday, Viollet had not decided who would be the fourth starter on defense. The club also is trying to sign another player to help defensively.

All of the uncertainty prompted midfielder Gary Darrell to note that the Dips will be fielding a team that is "just learning each other's first names."

Viollet, aware of the problem of familiarity, thinks he solved it.

"I departed from the normal practice pattern this week and went to a lot of game scrimmaging so the new and old players could learn each other," said Viollet. "Remember these guys are pros who don't need a tremendous amount of time together to play well together.

"I'd like to come away with a victory but if we play good, attractive, attacking soccer and leave the field with pride, I'll be satisfied."

Washington finished 2-5 in the exhibition season, using basically a makeshift lineup of rookies, free agents, walk-ons and a few veterans.

Newly acquired striker Peter Silvester netted four goals in the seven games and that alone should help ease the loss of last year's leading scorer, Paul Cannell. The English forward, along with three other 1976 starters, did not return this season.

Five players will be starting their first game in a Washington uniform.

Darrell, looking for his first goal since the 1975 season, player-coach Alan Spavin and quick Leroy DeLeon probably will handle the midfield duties while Silvester, English speedster Jim Refern and rookie Sonny Askew from Essex CC in Baltimore, will start at the forward spots. Eric (Goldfinger) Martin will start in goal.

"Of course, we'll be missing a couple good players but we can't worry about that," Viollet said.

Washington finished sixth in goals scored with 46 and 11th in goals allowed with 38 last year. The Dips also topped this season with 126 points, fifth best in the NASL a year ago.

The Metros (15-9) may experience some difficulty attaining the level of success they enjoyed last season. The club is picked to finish no higher than third in the Northern Division by the soccer experts.

Not one of the more offensive-minded clubs in the league last year (10th in scoring with 38 goals), Toronto hardly helped its cause by allowing talented forward Rusebio and midfielder Wolgfang Sunholz to play out their options and sign with Las Vegas.

The Metros split their only two preseason contests, losing to Tampa Bay, 1-0, in a tiebreaker shootout and beat Victory of Haiti, 9-0. Ivair Ferreira, the flashy 32-year-old Brazilian striker, tallied three goals in the win.

Defense carried Toronto to its NASL title. First-year coach Ivan Sangulin is hoping he can get the same aggressive, mugging style of play that helped the team lead the league in goals allowed (30), last year.

Bob Iarusci, mladen Cukon, Ted Polak and Filip Blaskovic are the backbone of the defensive unit. Paolo Cimpiel finished the season as the third leading goalie but ran into the contractural problems with the front office and missed the playoffs. His back-up, Zaljko Bilecki, played superbly, allowing just three goals in Toronto's four playoff games. His final two efforts resulted in a 2-0 shutout over Tampa bay and a 3-0 whitewash of Minnesota in the final.