The two teams drew 0-0 at San Siro last week in the first leg but the former Spain international is anticipating goals in their return meeting at Camp Nou.
"I take it as a given that Milan will score against us,” Guardiola admitted during his pre-game press conference on Monday.
“That's why we need to be very sharp on the attack and make the most of the chances we create.
"It will be a tough game. Milan are strong at the back and their counterattack is a concern, with [Zlatan] Ibrahimovic and [Kevin-Prince] Boateng.
"We have to play at a high tempo, with intensity and to the best of our abilities."
It emerged on Monday morning that influential Blaugrana midfielder Xavi was a major doubt for the visit of the Rossoneri because of a muscular problem but, according to Guardiola, the World Cup winner is in contention to start.
"He's been named to the squad; that means he could play," the 41-year-old coach stated.
"Xavi's got some fitness problems, but I am optimistic and I think the problem will not prevent him from playing.
"He will definitely play if he is fit but if not then someone else will take his place."
Barca's second-leg showdown with Milan gets underway at 20.45CET.
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