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Lionel Messi simply 'unstoppable' when he has room to operate - Max Allegri

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Barcelona 3-0 Juventus: Barca get revenge (2:42)

Barcelona and Lionel Messi dealt swift justice to the team that knocked them out of last year's competition. (2:42)

Juventus boss Massimiliano Allegri admitted that Lionel Messi is simply "unstoppable" after his team's 3-0 Champions League opening loss to Barcelona on Tuesday.

Messi took his tally to seven goals in his last three club games with a brace as Barca opened their Champions League campaign with an impressive win over last season's beaten finalists Juventus.

Following on from his derby hat trick against Espanyol on Saturday, Messi opened the scoring on the stroke of half-time after a one-two with Luis Suarez before drilling in Barca's third goal in the 69th minute after Ivan Rakitic had doubled their lead.

Messi might well have been celebrating another treble but saw his 51st-minute effort strike a post.

And Juve boss Allegri, whose side lost to Real Madrid in last season's final and also to Barca in the 2015 showpiece, said after the match: "He shot three times and scored two goals and hit the post.

"If you give him space he's unstoppable. If we don't give him any space then we can limit what he can do.''

But with a long road ahead of them, Allegri was far from panicking over his team's spot at the foot of the Group D table following the away loss at Camp Nou.

"I don't think we're going to have a problem. When we play against teams of this level we cannot allow ourselves to switch off. That's the difference," Allegri told Mediaset Premium.

"When they have the ball close to goal they are very dangerous. We have to improve and be more solid mentally.

"The lads had a good game, but when you have chances here, you've got to take them. We were punished for the first error, 30 seconds from half-time, when we didn't close Messi down or foul him.

"After the break we immediately had a great chance with Paulo Dybala, but when the spaces open up, Barcelona become devastating.

"These games have to be played with the mind. Paradoxically, we conceded the first and third goals on counter-attacks when we were trying to go forward.

"It is about having the strength to stay in the game to the end, because you can always get back and even turn it around."

Juventus host Olympiakos in Turin on Wednesday, Sept. 27, where the Serie A holders will look to earn their first points of the group stage in Europe's top club competition.

Information from PA Sport was used in this story.