BOSTON (AP) -- Paul Pierce and Kevin Garnett each had a double-double, and the Boston Celtics responded to yet another blow to their lineup with a 97-84 win Friday night over the Orlando Magic, who lost their eighth straight game.    Pierce had 14 points, 11 rebounds and seven assists despite sitting out the fourth quarter. Garnett finished with 14 points and 10 rebounds for the Celtics, who learned early in the day that promising rookie Jared Sullinger will miss the rest of the season following back surgery.    The short-handed Celtics also lost All-Star point guard Rajon Rondo this week, yet won their third straight and got back to .500 at 23-23.     Courtney Lee had 14 points and five assists for Boston. Jeff Green scored 17 points.     J.J. Redick had 15 points and five assists and Nikola Vucevic had 14 points and 14 rebounds to lead Orlando, which is dealing with injury problems that rival those of the Celtics. The Magic were without their top two scorers in Aaron Afflalo (calf) and Glen Davis (broken left foot), then lost point guard Jameer Nelson to a bruised left forearm. Nelson, who averages 7.4 assists and is third on the team in scoring at 15.3 points per game, did not play in the second half.    The depleted Magic still managed to push the Celtics through much of the game, taking advantage of 15 turnovers and some shaky free throw shooting as Boston went 9 for 14 from the foul line.    Orlando had been chipping away at the lead in the third quarter, pulling to 57-54 on E'Twaun Moore's three-point play with 5:04 left in the period. Boston slowly rebuilt the lead with a few short runs, then broke open the game in the fourth when the weary Magic wilted defensively.    Pierce scored the last four points of the third quarter, putting the Celtics up 72-59 and starting a 10-0 run for Boston. The Celtics opened the fourth quarter 7 for 9, including a series of dunks by Lee and Green during an 8-2 spurt that extended the lead to 86-63.    Jason Terry and Leandro Barbosa scored 12 each for the Celtics, who had six players in double figures.    Andrew Nicholson and Moore finished with 14 apiece for the Magic.    NOTES: Boston has won seven straight regular-season games against the Magic. ... Boston had 30 assists. ... The Magic were 1 for 13 on 3-pointers in the first half. ... Orlando has not won since beating Indiana on Jan. 16.