An own goal, plus Paulo Dybala and Juan Cuadrado were on Juve's scoresheet.
“We have to defend better, as we concede too many goals and risked letting in an equaliser at the 85th minute," Allegri told DAZN.
“Defending like this is tiring and we have to work harder as a unit to prevent these situations. This just isn't good enough. Even after the first goal we didn't add a second and drifted out of the game. We can't always assume that we'll score two or three goals."
Allegri revealed there was a new injury concern for Douglas Costa, who made way for Cuadrado at half-time.
“He had some pain in the groin area and it was better not to risk it. Cuadrado did really well, as did Rodrigo Bentancur in midfield. We knew the Uruguayan had quality and just needed a bit of time.
“The most important thing now is for everyone to improve our defensive movements, because we need everybody to contribute."