Despite leading by three points with 18 seconds remaining, a couple of turnovers handed the game to the Raptors, who sealed the win thanks to a late jump shot from Patrick Patterson.
Garnett was eager to praise Paul Pierce's display after the veteran scored 33 points, but conceded that the Nets needed to improve in the closing stages of games.
"Paul played well," Garnett told reporters at the Barclays Center.
"He totally carried us for that fourth quarter, that second half, and we all came in and just said at the end of the game we've got to make plays, we've got to close games out and tonight we didn't do that."
Brooklyn have lost just twice in 2014, with both defeats coming against the Raptors.