The high-scoring Arenas was acquired from Washington and finished with 10 points and three assists in 27 minutes of action. Jason Richardson, Hedo Turkoglu and Earl Clark were acquired from Phoenix on Saturday.
Richardson (nine points, three assists) and Turkoglu (eight points, six rebounds) started for Orlando (16-11) while Clark did not play . The NBA certified the trades less than an hour before the tipoff.
Arenas came off the bench because Magic coach Stan Van Gundy says he wants to keep guard Jameer Nelson as a starter.
Orlando called off its shootaround Monday because the four new players had to take physicals. Van Gundy says all four passed the physicals.
"We're starting from scratch," said Orlando coach Stan Van Gundy. "We would have all liked it to have been better but it wasn't."
"For being together a couple hours, we did all right," Arenas said after the loss.
Hawks guard Jamal Crawford missed his fourth straight game with a sore back.
Information from The Associated Press was used in this report.