Massimiliano Allegri bemoaned a collapse in the defensive standards that humbled Barcelona five months ago after Juventus's 3-0 UEFA Champions League Group D defeat at Camp Nou.

Juve famously shut out Barca over the course of 180 minutes to win 3-0 on aggregate in last season's quarter-final, but the LaLiga leader found its Serie A counterpart a far less robust proposition this time around.

A brilliant Lionel Messi goal put the host in front towards the end of a first half as even as the second period was one-sided.

Messi created Ivan Rakitic's goal and helped himself to another as a Juve defence without the injured Giorgio Chiellini and still adjusting to life without Leonardo Bonucci provided increasingly flimsy resistance.

"We played a good first half but when you play these kind of games you have to take your chances," Allegri said. "We didn't and we were punished at the end of the first half. Ironically we conceded three goals from counter-attacks but this was our worst mistake because we had to remain compact after Barcelona's goal.

"Obviously they have this player who can change every game but we had to do better in the second half.

"You cannot expect to come here and dominate the game so we have to play better at a mental level, always trying to remain in the game even when everything seems difficult.

"If you leave spaces to Messi, you get punished."

It meant a Champions League debut to forget for Rodrigo Bentancur, who earned a maiden Juventus start.

The Uruguayan midfielder, who cut his teeth with Argentine giant Boca Juniors, was left to marvel at Messi's otherworldly talents.

"Messi is incredible, he is a player from another planet," Bentacur said. "We are sad for the defeat but we gave everything we had. Unfortunately it was not enough.

"I'm happy with my performance, the Champions League is different from the Copa Libertadores, there is more pace and better  technique.