Allegri's men travel to Spain for a UEFA Champions League Group H blockbuster against Lionel Messi and co on Wednesday.
They held Barcelona to a 2-2 draw in the San Siro two weeks ago but the coach admitted repeating that feat at the Nou Camp will not be easy.
"First of all, tomorrow we are going to face Barca for the eighth time in three years," Allegri said.
"It's an important game of course and it will be very tough for us as Barcelona are still the best.
"I think that tomorrow, if we want to leave with a positive result, we'll have to do an excellent job as a whole unit because playing against them, it's normally very difficult, so we'll have to stay aware.
"If we play well both in defence and in attack, just like we did in the first leg, we'll have some chance.
"We don't have to play the sacrificial victim but we need to know that we are going to play against the best in the world.
"We got a good result two weeks ago in Milan, but also two seasons ago we drew here 2-2 in what was definitely our worst game against them, we also drew 0-0 at the San Siro.
"As I said, playing against them, it's always tough as they are the only ones in the world that really wear you out with their style of play, both physically and mentally.
"So we will have to play well like we did in Milan."
Milan goalkeeper Christian Abbiati hopes the side's purple patch of form can continue against Barcelona and said these games are the reason he plays football.
"I think that every footballer wants to play a game like this in the Champions League," Abbiati said.
"For us, it's a very important game as we are in a very good position in the group at the moment and we want to keep that.
"And I think this game comes round at exactly at the right moment - we are not going through a happy time at the moment, so this game is the kind of match that can give you the incentive to do well."
