There appears to be an 80 percent certainty that the Washington Diplomats will remain in Washington next season even if, as expected, they are sold to Madison Square Garden Corp. next week.

According to league sources, the Garden group, headed by David (Sonny) Werblin, initially wanted to move the team to New York's Shea Stadium, but has virtually decided to keep the team here.

The North American Soccer League, which would have to approve both the sale and the shift, if requested, reportedly is eager for the Garden to purchase the club from current owner San Juan Racing Inc. The league will approve the sale under any circumstances, the sources said.

Although the SJR board does not meet until next month to decide the fate of the team, technically it has already ordered club President Steven I. Danzansky to try to close the deal prior to the board meeting.

Among details to be worked out, the most important is the lease with the D.C. Armory Board, which operates RFK Stadium. The sale is expected to be completed by Tuesday and approved by the league at its Chicago meeting Thursday.

Although the league wants to keep a club in Washington sources say it will accept Garden ownership under any circumstances.

Under SJR's ownership the Dips reportedly have lost nearly $2 million in three seasons. Last season the team made the league playoffs with a 16-14 record and averaged little more than 10,000 fans per home contest.

Garden Vice President Jack Krumpe said earlier in the week that his group would seek increased attendance through a "different" marketing approach as owners of the Diplomats.

Sources said that the "different" approach would parallel the Cosmos' successful method of drawing fans: buying top-flight world class players.

The Garden, if it buys the Dips, is expected to spend lavish sums in the next few months, trying to field a team competitive with the New Jersey based Cosmos. Winners of two straight NASL titles, the Cosmos averaged more than 50,000 fans per home game at Giants Stadium this season.

No immediate shake-ups are expected in the club's front office. Garden owners would "look at the organization with a microscope," for several months, according to one league officials. Then they would decide on changes. Wholesale player changes are likely.

The Garden group probably would keep the club in Washington for two major reasons: first, it would avoid paying the Cosmos a $5-$7 million indemnity for invading their New York territory. Also, Werblin appears convinced that Washington will accept soccer if it is marketed properly.

There is also a possibility that the Garden could acquire a baseball team for RFK Stadium, along with the Diplomats, a league official said.

The Dips have been in Washington since 1974. They moved to RFK Stadium under the SJR ownership in 1977 and averaged more than 13,000 fans - many nonpaying - during a 10-16 season.

Last year "paid attendance doubled with a winning team but "total" attendance dipped. It was first reported that the Dips were for sale in July. Negotiations with the Garden began in early September.