Harris proceeded to drain the key foul shots, providing the winning margin as the Magic triumphed for the first time in 2014.
Harris finished with 18 points and seven rebounds.
Arron Afflalo supplied 20 points, 13 rebounds and six assists for Orland, and Glen Davis added 17 points to go with seven rebounds. Nelson helped out with 16 points and 10 assists.
The Celtics fell for the 11th time in 12 games despite 22 points from Jeff Green.
Humphries and Avery Bradley netted 18 each.