The NBA has cast doubt on the proposed trade between the Clippers and Boston Celtics because a coach cannot be traded for a player under the collective bargaining agreement, said a person familiar with the talks who was not authorized to speak publicly on the matter. The deal that has been discussed and was close to being completed was Boston sending the rights to Coach Doc Rivers and Kevin Garnett to the Clippers for DeAndre Jordan and two first-round draft picks. The person said that even if the Clippers and Celtics break the trade into two transactions, the NBA would frown upon that because the Clippers are trying to get Rivers more than anything else. “They are chasing the Tooth Fairy,” the source said. The NBA has let the Clippers and Celtics know for “several days” that it was a deal that couldn’t be done as presently put together. NBA commissioner David Stern addressed the deal Thursday on ESPN’s national syndicated radio show “The Herd with Collin Cowherd.” Stern agreed on the show that NBA rules prevent trades with “contingencies or side deals attached.”