Harkless and Tobias Harris combined for 16 points in the fourth quarter Sunday night and the Magic beat the Celtics 103-98.
"It felt a little weird to be out there playing," said Harkless, who played 30 minutes after averaging fewer than 14 minutes and sitting out more than half of the Magic's games this season.
"To play so little all year and not know when you're going to play, it can be tough to be ready," he said. "But these guys are always encouraging me and they believed in me. I tried to have fun and played basketball the way I know how to play."
Harkless started a decisive 12-0 fourth-quarter run with a 3-pointer and scored nine of his 14 points in the final quarter.
"We kept telling him to stay ready and tonight was his night," Magic coach James Borrego said.
"He's done a great job of continuing to get better while he wasn't playing," said Magic guard Victor Oladipo. "Tonight his opportunity came and he did what he needed to do."
Oladipo scored a game-high 22 points for the Magic, who trailed by 20 early before recovering to win their second straight. Harris finished with 21 points and rookie Elfrid Payton had 19 points, 10 rebounds and seven assists.
Isaiah Thomas scored 21 points to lead the Celtics. Brandon Bass added 19 points and 16 rebounds, but Boston shot only 35 percent after the first quarter.
"I think anytime you play a good one quarter and get crushed in the other three, it's probably not going to be a good night," Boston coach Brad Stevens said. "And I thought we got crushed in the other three."
After winning nine of their previous 14 games, the Celtics damaged their slim playoff chances by losing to the Magic for the second time this season.
"If we want to be good," Stevens said, "we've got to play a lot better than that for four quarters."
Dewayne Dedmon, starting in place of injured Magic center Nik Vucevic, had career highs of 11 points and 16 rebounds.
Coming off back-to-back games of 38 and 32 points, Oladipo had only two field goals in the first half and only seven points before converting a three-point play with 7:30 left in the third quarter.
After the Magic shot 6 for 21 with seven turnovers in the first quarter, Evan Turner and Jonas Jerebko scored the first six points of the second quarter, opening a 37-17 lead for the Celtics.
The Magic never led until Oladipo's reverse layup made it 61-59 late in the third quarter.
Celtics: Boston shot 39 percent from the field, but that was boosted by its 44-percent effort on 3-pointers (15 for 34). The Celtics were 20 for 55 on 2-point shots. ... Boston's reserves outscored Orlando's backups 40-24.
Magic: Orlando played a second straight game without center Vucevic and guards Willie Green, Even Fournier and Luke Ridnour. ... The Magic are 6-6 under interim coach James Borrego.
Celtics: At Miami on Monday night.
Magic: At Indiana on Tuesday night.
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