THE SIDELINES : Boston Celtics Sign Dino Radja

The Boston Celtics, taking the view that 6-foot-10 Dino Radja was an amateur with his former Yugoslavian basketball club, have signed him to a one-year contract in the very professional National Basketball Assn.

“I feel very nice now,” said Radja after his first official NBA workout with the Celtics at Brandeis University on Tuesday, but he admitted that he felt bad that his Yugoslavian team, Jugoplastika, had been unsupportive and uncooperative in his pursuit of a basketball career in the NBA.

The Yugoslavian club maintains that Radja signed a four-year pact and is committed to it through 1992.

Celtics General Manager Jan Volk disagrees. “Their contract is an equivalent of an amateur contract,” he said.

Terms of his contract were not made public, but Radja said he feels like “the richest man in the world.”