The Rossoneri earned a 1-1 draw at San Siro and visit the Camp Nou tonight.
"This is an important match where we will run into a lot of problems, as Barça have always been a strong side," said Allegri.
"It will require a great performance, both physically and psychologically, both in attack and defence. We need to be wary, but also have courage to play and do even better than in the first leg. Last year our 4-0 defeat here was our worst game in recent campaigns."
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