Neil Lennon has said that beating Barcelona in the Champions League would take a "huge effort" from Celtic.
The Scottish champions, who beat the La Liga outfit at the same stage of the tournament last season, host Gerardo Martino's men on Tuesday.
The Hoops manager told Sky Sports News: "We beat Barca last year, we'll have to play again at that level of intensity, concentration, defend well, maximise the opportunities that we get when they come.
"I think Barcelona again have started the season very, very well. I think they'll be hurting from going out at the semi-finals last year, I think there's some players there who still have to prove themselves at Barcelona at that level which is the highest level.
"They have a new manager obviously but the infrastructure behind is still in place and of course they have got Messi who is a very good player. It's going to be monumental, it's going to take another huge effort but you never say never."
Celtic lost their opening game of the group stage against AC Milan earlier this month.
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