The Baltimore Orioles learned today that the knee injury to their unbeaten left-hander, Mike Flanagan, was worse than they had hoped, yet better than they had feared.

Flanagan will not require surgery and will not be lost for the season. However, he will be sidelined for eight to 10 weeks with an "incomplete tear of the medial collateral ligament." The pitcher with a 6-0 record, who hurt himself Tuesday night while lunging and twisting to field a ground ball, will be in an "immobilizer" for at least two weeks, then switch to a special brace to begin therapy.

At this point, the Orioles' best guess is that Flanagan might be ready to pitch soon after the All-Star break, thus missing perhaps a third of the year.

"Eight to 10; it sounds like a jail sentence," said the Orioles' Ken Singleton.

"Two months is long enough, but it could have been a lot worse," said General Manager Hank Peters, who, like most of the team's brass, didn't know whether to be depressed by losing the team's most effective pitcher or heartened that the former Cy Young Award winner would almost certainly be back by Aug. 1--in time for the heat of the pennant race.

"It is a terrible blow to us," said team owner Edward Bennett Williams. "Mike's a fast healer, but you don't know how quickly he'll actually be able to pitch. The real shame is that this looked like it was going to be his finest year, better even than '79."

Since Jim Palmer, on the disabled list, is not expected to pitch for at least another week, the Orioles plan to go with a five-man rotation of Dennis Martinez, Scott McGregor, Storm Davis, Mike Boddicker and Sammy Stewart, who will start Saturday against Toronto.

The Orioles have also purchased the contract of left-handed reliever Dan Morigiello, who reported to the team for tonight's game, to get another left-hander on the staff in the bullpen.

"We thought we were so deep in pitchers that we were going to try to trade one over the winter for an infielder," added Williams. "Oh, boy, am I glad we didn't. We need Jim (Palmer) back and for one of our youngsters to come through. But we could still be all right."

The Orioles were unable to hide their hopes that Flanagan will mend more quickly than predicted. "Doctors make their estimates (of healing time) based on their experience of normal people," said Pitching Coach Ray Miller. "I don't think these guys are normal . . .

"He's not in a cast. He'll be able to walk around in the immobilizer," continued Miller. "He's been pitching with a torn groin muscle. I've seen him pitch a whole year with tears in his ankle and another year with a hole (muscle atrophy) in his pitching shoulder. Mike has about as much tolerance for pain as anybody in the game."

The Orioles' mood was brighter than last evening, when many assumed Flanagan, who was unavailable for comment yesterday, would be lost until 1984.

Flanagan's close friend, McGregor, wasn't quite so cheerful after hearing word of the results of the arthrogram performed on Flanagan today.

"I still feel like, in a way, his whole year's been wiped out," said McGregor. "I just hope he comes back when he's ready to come back. Leg injuries lead to arm injuries. Mike has come back too soon from just about every injury of his career and he's paid for it (in shoulder miseries). He can't afford to do the same thing this time. It's his (left) pushoff leg and the whole weight of your body is on it."

Manager Joe Altobelli, trying to look on the bright side, said, "It's an old baseball cliche, but it's still true that when you have an injury, you never know what you might fall into. Wally Pipp had a stomach ache and the Yankees found Lou Gehrig. We've got some good young pitchers and this will give them a chance."