ALTHAM - The NBA had hoped to announce yesterday the participants in next month's Slam Dunk Contest on All-Star Saturday. It had to resort to Plan B, however, when it learned that Celtics rookie Kedrick Brown will be unable to participate because of an ankle sprain.

Celtics coach Jim O'Brien spilled the beans after a brief workout yesterday. He said the team had been led to believe that Brown was on the short list and that '' Antoine [ Walker] and Paul[ Pierce] would have made sure he participated.'' All that was missing was the official notice from the league office.

But Brown rolled his right ankle in the first quarter of Wednesday night's 98-94 victory over Indiana. It did not look good on television and it did not look good to team doctors or trainer Eddie Lacerte. The prognosis: Brown will go on the injured list and be out for more than two weeks.