Dave Cowens' sojourn from basketball ended yesterday as unexpectedly as it began.

Two months after he took an unpaid leave of absence from his $280,000-a-year-job as the Boston Celtics' center and two days after he started work as a race track executive, Cowens reversed his field.

He announced in Boston that the would abandon the assistant general managership of New England Harness Receway and rejoin the world-champion Celtics, who have been anything but lordly without him.

"My enthusiasm has returned and I'm anxious to get started playing again," said the 28-year-old Cowens, who claimed the fire had gone out of him when he left the Celtics Nov. 8.

But the reasons for his return run far deeper than that. They have to do with the pressure he has been under since making his original decision and with Celtic general manager Red Auerbach's distaste for Cowens' involvemnet with the race track.

Both subjects were batted around thoroughly yesterday afternoon when Cowens met with Auerbach and team owner Irv Lewin for the meeting that ended professional sports' longest-running soap opera.

"I told hin I didn't think the harness track was the place for Dave Cowens to work," Auerbach said later. "I respect and like the people who own the particular track Cowens was at, but I still felt he shoudn't be there."

Auerbach had a fatherly explanation for his stance. "There are scandals throughout the (harness racing) business," he said, "and I don't think it's proper atmosphere for Dave."

But the man who built the Celtic dynasty as a coach also was not above letting economic factors enter into his pitch to Cowens.

"For him to be in the race-track business while we're playing basketball in all probability was a violation of his contract," said Auerbach. "We are in competition with the track, and I told him of the conflict."

No doubt that argument added to the pressure Cowens was already feeling.

"I was tired of taking as lot of flak from many circles," he admitted after his great leap backwards. "It was netting to me. I didn't like hearing that I was hurting the team."

When he was still weighing his decision and he told Auerbach that, the stubby, cigar-smoking general manager wasted no time moving in for the kill.

"If you're tired of people bugging you," Auerbach recalled telling Cowens, "come back to work for the Celtics now."

Cowens said he would, and Auerbach, who had been meeting with the 6-foot-9 droupout "every so often," heaved a sigh of relief.

The Celtics, who are 19-18 after winning their 13th NBA title last June, have not been their fearsome selves since Cowens departed, no matter how loudly Auerbach has argued the contrary.

Indeed, less than two weeks ago, Auerbach indicated just how bad off the Cowens less Celtics were by visi- ting then on a West Coast swing, a trip he makes about as often as he visits Sri Lanka.

What he saw was not pleasant. First, Golden State whipped Boston 105-98, as Cowens' stand-in, Jim Ard, accounted for just two points. Then the Celtics traveled to Portland and were destroyed, 128-84, while the hapless Ard scored nary a point.

So Auerbach will be delighted to have Cowens back in uniform for Friday night's return bout with the Trail Blazers, even though there is a legitimate question about what kind of shape the red-haired center is in.

"It's up to him how much he plays," Auerbach said. "He's back with the ball club and we couldn't be happier. It will be up to him if he wants to play 30 seconds or five minutes."

"I may not be in shape until next September," joshed Cowens, who has stayed at his playing weight of 230 pounds with a regimen of light workouts.

According to most accounts, the Celtic players will be ecstatic about Cowens' return. But there was one incident shortly after he left the team that could be the source of friction.

When Cowens came to Boston Garden to see Don Nelson's number retired, Jo Jo White was quoted as cursing him for showing up his teammates.

But Cowens refused to put any stock in White's anger. "It was something Jo Jo said on the spur of the moment," he insisted. "It didn't mean anything."

With that, Cowens started preparing to play basketball, as he put it, "for the rest of the season and forevermore."