Pagliuca said Friday he was driving and talking on the phone with Celtics president of basketball operations Danny Ainge when he got the news.
"I almost crashed my car when I heard that," he said, according to Tom Westerholm of MassLive.com.
Walker inked a four-year, $141-million deal with the Celtics as part of a sign-and-trade deal with the Charlotte Hornets, the franchise with which he spent the first eight seasons of his career.
"Kemba is really excited," Pagliuca said. "I always like to watch him play unless it was against us. He beat us last year single-handedly in the last five minutes, so it's great to have him on our side. Just really positive attitude from everybody this year."