He says he feels like a rookie groping his way along. Sometimes he looks that way, too. But Dave Cowens is loving every minute of being a basketball player again.

Cowens, the National Basketball Association's most valuable player as a center for the Boston Celtics in 1973, played his third game as a Milwaukee Buck tonight against the Washington Bullets in the first game of a Madison Square Garden doubleheader. The Bullets won, 112-99 (New Jersey beat New York, 93-83, in the second game), but Cowens, now a power forward, showed a little of why Coach Don Nelson parted with Quinn Buckner to obtain the former Celtic, even though he hadn't played a game in two years.

Cowens, slimmer and trimmer, still banged around with Bullets Jeff Ruland and Rick Mahorn, but he also went outside to exhibit a little more finesse than the old Cowens might have.

He played 29 minutes and made five of seven shots from the field and had five rebounds, an assist and a steal.

The Bucks, playing without Marques Johnson, Junior Bridgeman, Brian Winters and Paul Pressey -- all sidelined with injuries -- were overmatched against the Bullets as Greg Ballard scored 17 points and Ruland 16.

Ruland, Dave Batton and Charles Davis fouled out and the Bullets committed 43 fouls. They still played far better than in their five-point loss to Atlanta in their first exhibition game Monday.

Davis made all six of his field goal attempts and Ruland was six for seven. The Bullets led virtually the entire game.

But the predominantly Knicks fans at the Garden were more interested in Cowens.

"He's way ahead of the schedule we had in mind," said Nelson, a former Celtic teammate of Cowens. "We still want him to be a little more aggressive than he has been so far, but he's adjusted to playing a new position very well. Sure it was a gamble to give up someone as valuable as Quinn to get him, but obviously I felt it was a gamble I needed to take."

Cowens still uses some of his old moves, like the left-handed jump hook from the right side of the basket, and he still is deadly from the corners.

"It's just a beginning, though," he said after tonight's game. "I still have a long way to go. I'm learning a new position and I'm learning to play with new people. I have to get a whole new book on everybody in the league, too."

Nelson wants Cowens to provide the Bucks with more muscle inside to take some of the physical load off center Bob Lanier.

Cowens, 6 foot 8 1/2, probably is more of a forward than a center anyway, but he played the middle in Boston out of necessity. However, the transition to power forward hasn't been all that easy.

"It takes a lot of repetition to learn to do things right," he said. "The biggest difference playing forward defensively is that you have to guard a man 18 feet from the basket sometimes. At center, you were always in the middle, giving help to your teammates. At forward, you're getting picked all over the place and you're the one who needs the help."

Because Cowens retired abruptly nine days before the start of the 1980-81 season, after having previously walked away from the Celtics before in the 1976-77 season, the Bucks were taking another chance that Cowens might become disenchanted again and take another walk.

Nelson says he isn't worried about that and Cowens, too, said it won't happen.

"They (the Bucks) gave up a great player to get me," he said. "I'm here for the duration."