“That’s the improvement and growth we’re looking for,” said Cavs forward Larry Nance Jr., who added 18 points and 10 rebounds.
Cleveland played with a lead all night, but the Cavs had to hold off a second-half rally by Boston led by Jayson Tatum and Jaylen Brown.
Tatum scored 29 points and Brown 28 for the Celtics, whose only lead was at 2-0.
With Cleveland clinging to a four-point lead, Garland hit a runner in the lane, and after a miss by Brown, Cavs rookie Isaac Okoro took a pass by Garland and threw down a vicious dunk and was fouled by Tatum.
Okoro had 15 points, including seven in the final quarter to help the Cavs hold on.