The AP reports: Sam Cassell and Boston’s backups outplayed Atlanta’s starters in the fourth quarter, and the Celtics prevented the Hawks from clinching their first playoff berth since 1999 with a 99-89 victory Saturday night… Cassell, 38, scored 20 points, including 15 in the fourth quarter, as the Celtics outscored the Hawks 21-5 down the stretch… Kevin Garnett sat out the fourth quarter but led Boston with 24 points. Pierce and Ray Allen each had 14, also in three quarters… Joe Johnson scored 21 and Smith and Horford each had 17 for Atlanta. Bibby added 16.