The game has been given plenty of hype as the two European giants fight for supremacy in Group H, but the Italian is not getting carried away with the build-up, preferring to focus on the tactical task of besting the holders.
"Living these games is the stuff of dreams for everyone at Milan," Allegri told the press.
"Barcelona are the strongest team in the world, but we have to try to beat them by taking full advantage of the chances that we will create.
"When we played in Barcelona [in matchday one] we created plenty and we have to try and do the same."
Allegri has been analysing Barca boss Pep Guardiola's style of play and said it will be hard to dominate the reigning champions.
"It's difficult playing against Barcelona because they have technique and they always vary their ryhthm in matches," the Italian reasoned.
"We will need to be balanced and play higher up the pitch so as not to allow Barcelona too much possession."
A win will see Milan move above Barcelona in the group ahead of the final fixtures.
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