Incentive is the last thing the Washington Diplomats should need in tonight's 8 o'clock match of RFK Stadium against one of the North American Soccer League's hottest team, the Portland Timbers.

The Dips. 10-9, should remember the National Conference's Western Division-leading Timbers, 13-5, as the culprits who started them on their way to a club-record six straight losses with a 2-1 shootout victory in Portland May 24.

That victory also was the fifth of what was to reach a NASL-record nine consecutive victories for Portland. The streak ended with a 2-0 loss in Tampa Bay Wednesday night.

The Dips would like nothing better than to help the Timbers continue in that direction and at the same time strengthen their hold on second place in the Eastern Division.

Washington is feeling confident again,not because the Timbers lost their last outing and two of their top players - forward Willie Anderson and midfielder John Bain - but because its dormant offense came alive against Los Angeles Tuesday night.

Washington embarrassed the Aztecs, 4-0, as Paul Cannell scored two goals and Cary Darrell and Ken Mokgojoa each added a goal.

"We played with confidence against Los Angeles," said Washington Coach Gordon Bradley.

That victory was especially graftifying for Bradley and the Dips since they had been trounced by the Cosmos, 6-1, two days earlier. But Washington, which suddenly has taken a hard look at the division as well as the other playoff contenders, realizes with just 11 games (including tonight's contest) remaining in the regular season, every game is essential.

"We have to win our home games or it'll be tough," said Bradley. "With Rochester and Toronto breathing down our necks, we must win to keep our advantage over them."

Following tonight's game, Washington hosts another division leader - San Diego - Wednesday night before hitting the road again for three of its next four games.

Portland doesn't have any players listed among the league leaders but only six teams have scored as many goals. Striker Clyde Best and winger Stewart Scullion have accounted for 12 of the Timbers' 30 goals. Best also has seven assists to go with his six goals.

Defensively, the Timbers rely on Mick Poole, the NASL's seventh-ranked goalkeeper. Poole has allowed only 22 goals in recording five shutouts. He has more than 100 saves.

Washington, winner of eight of its first 10 games before injuries and a lack offense sent it on its six-game losing streak, has begun to show some continuity on the field, especially in the midfield.

Much of that credit must go to newly signed Dutchman Cus Hiddink, who has settled the offense and even chipped in with his first goal in the Cosmos' game.

"Hiddink has really helped us in the midfield. That's one reason I was able to switch from a 4-2-4 to a 4-3-3. With him controlling the ball, I can send more people on the attack without losing too much in the back," said Bradley.

"We lost a tough shootout decision to the Timbers and we had our chances to win that game," Bradley said referring to the May 24 game. "We don't intend to let this one slip away from us."

Goalie Bill Irwin, who jammed his thumb against Los Angeles and sweeper Jim Steele, still hampered by a pulled groin muscle, are expected to start tonight . . . Cannell with his 10th and 11th goals of the season, has tied his season and career scoring mark of 28 points . . . Portland is 5-0 in shootout games . . . The Dips released reserves Pete Caringi and John Borozzi yesterday. Neither had seen any playing time . . . Washington is averaging 11,055 in attendance for nine home dates.