As if 13 weren't enough.

The Boston Celtics added another flag to their collection last night-the 14th for the franchise, but the first for the Bird et al-with a gutsy 102-91 win over the Houston Rockets.

Champagne flowed like Gatorade in the Celtic dressing room after the game, but for a large part of the fourth quarter it looked as if the Greenwould have to return to the Garden to wrap it up.

Up by 17 as the final stanza began. Boston watched helplessly as the Rockets whittled the lead down to three. But Celtic coach Bill Fitch called a pair of key timeouts, the Bostons sat down and talked things over, and Mr. Bird decided to take control.

The next time down the floor Bird threw in a rainbow turnaround: under the other basket he drew a charge and then hi one more jumper. The Celts were up by seven: not out of the woods but no longer asleep.

The Bird surge broke Houston's back. After the Rockets once more closed it to five, it was their turn to watch helplcssly while the Celts handled the ball with grace and sank the freethrows awarded them by a steady string of Houston fouls.

Until the fourth-quarter Houston spurt, the Celtics had controlled the contest. Even without the services of Robert Parish, sidelined in the first quarter with three fouls. Boston built a seven-point halftime lead, largely on the sterling offensive play of series MVP Cedric Maxwell.

The happiest man of all may have been Red Auerbach, sitting in the box seat who for the 14th time in his life got to light the championship Havana.