A Boston club playing without Paul Pierce due to a right knee injury also lost Garnett late in the game when he leaped to block a Vince Carter shot, fell awkwardly to the court, then rose and hobbling to the bench holding his back.
Garnett, who missed his first game of the season on Tuesday with a thigh bruise, had 10 points and seven rebounds.
Ray Allen scored 18 points and Tony Allen, replacing Pierce in Boston's lineup, added 16 as the Celtics improved to 23-5 with their 14th triumph in 15 games.
Vince Carter scored a game-high 27 points in a losing cause while Rashard Lewis added 19 points and Frenchman Mickael Pietrus added nine for the Magic, who fell to 22-8, still one game ahead of Atlanta atop the Southeast division.
Magic All-Star center Dwight Howard, who pulled down 20 rebounds, was kept to only five points as Orlando managed only eight points in the second quarter, trailing 38-27 at half-time.
Orlando, which had won seven of the past 10 meetings with Boston, trimmed the deficit to 78-75 before Ray Allen's jumper and two free throws pushed the Celtics out of range for good.
The matchup between the two top clubs in the Eastern Conference was among five Christmas Day games on the NBA schedule, something that irked Orlando coach Stan Van Gundy.
"I actually feel sorry for people who have nothing to do on Christmas Day other than watch an NBA game," Van Gundy said. "If I had my way, we'd take a five-day break at Christmas.
"Obviously basketball is very important to me, but there are some days of the year where it's got to take a back seat to something."
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