Usually, Scott McGregor is the Baltimore Orioles' professional optimist. He'd see a silver lining in a tinfoil bomb.

Tonight, however, after watching his teammates lose, 5-2, as his buddy Mike Flanagan was knocked out in the first inning without retiring a batter, even McGregor fell into a funk.

As McGregor left Fenway Park, someone tried to hand him a piece of encouragement: after the Orioles leave Boston, McGregor was informed, they get to play 19 straight games against the three worst teams in the American League.

"You mean we play ourselves?" McGregor said sardonically.

Four consecutive losses and 12 in its last 17 games have made the whole team just such a study in blue.

As Manager Earl Weaver said tonight, "The handwriting is on the wall . . . We've got to get our butts in gear and play better real soon. We could be overanxious, but you can't tell 'em not to try. You can say, 'Relax,' but it don't mean anything . . . We just have to wait and hope we come out of it."

When a team is playing well, it feeds off its own jubilation and confidence, rushing from day to day, victory to victory, certain of its skills and its good fortune. Because the game is daily, the psychic reinforcements are constant, the occasional defeats are easily forgotten, erased by the next game on the schedule.

But, when a team is losing, the way the Orioles are losing, the process reverses itself. Negative reinforcement is every bit as real as positive feedback. Perhaps it's more powerful since, in a hard world, many people tend to grow accustomed to expecting the worst.

Day after day, the litany of misfortunes, sins against self and wrongs done to you, grow longer. This evening, before 32,935 watching in misty, autumnal Fenway, the Orioles added to their heavy burden of guilt.

For the sliding Orioles, who now are closer to last place in the AL East, six games, than they are to first, seven, many a recent day has seemed like Friday the 13th. So, when the real thing came this evening, they had plenty of practice.

In a defeat that seemed to be a kind of apotheosis of their futility, Flanagan never got an out, relinquishing five scorching base hits and three runs. In other words, the badly slumping Red Sox, who had lost 10 of their last 13, got more runs before they made an out than the Orioles got off left-hander John Tudor the whole game.

Except for a homer by Lenn Sakata and an RBI triple by Ken Singleton, the third-place Orioles were almost soundless in this five-hit defeat that pushed them 2 1/2 games behind the second-place Red Sox. As an added, unbelievable fillip, Baltimore now has gone more than a month without a victory on the road; it is 0-8 since July 11.

These days, the Orioles will believe anything, even a no-out knockout.

"Never happened before and I hope it never happens again," said Flanagan. The 8-10 left-hander had 30 relatives and New England friends in the stands, yet lasted only 19 pitches. In fact, he threw only one strike that was not hit.

"He warmed up the best in a month," said Ray Miller, the perplexed pitching coach. "People were ooohing and aaahing in the bullpen. Then he comes to the mound and has nothing."

Flanagan, like many, wonders if this team is a genuine descendant of the 100-victory clubs of 1979 and '80, or if it is a drab duplicate of the '81 edition, which never got out of first gear.

"Maybe we don't know which we are, either," said Flanagan.

Weaver is usually at his best in just such refuse-to-panic situations. After 35 years, he knows that baseball situations seldom are as dire as they appear. In a sport of endemic streaks, a manager is always wise to anticipate what shape the future would hold if his team won, or lost, its next eight or 10 games. It happens all the time.

After Wednesday's loss of a three-game series in Chicago, Weaver kept the team bus waiting. When he finally entered the four-wheel sepulchre, he reportedly said (minus several adjectives), "If you want to know why the bus is late, it's because I've been in there explaining to some people how we're still going to win the pennant. It took a while, but I did it."

However, the telltale signs of gradual loss of faith--or, perhaps, simple reality--are appearing.

On a Washington to Boston plane today, team owner Edward Bennet Williams, who was here tonight, quipped, "I'm on a damage control mission."

Weaver and Williams had a 40-minute pregame meeting. Said Weaver, "We talked a lot about next year . . . not me, but about the ballclub."

When the manager and the owner talk about the shape of next year's team, it doesn't say good things about this year's chances.

This evening, the Oriole leaders quickly wished that their thoughts could have stayed in the future. Only Sammy Stewart's eight excellent, but futile, innings of long relief (two runs and six hits) was uplifting to the visitors.

Saturday is Weaver's 52nd birthday.

"I'm only asking for one thing," he said. "A win."

The Orioles, however, seem intent on giving him a more dubious present. They already have completed their season's series in both Kansas City (0-6) and Chicago (0-6) without a victory. Here in Fenway (0-3), they have a chance to add a third city to that shocking list.