The veteran midfielder believes the Italian champions cannot afford to entrust a player with the sole responsibility of tracking the forward's every movement for 90 minutes
The Italian champions managed to keep the Blaugrana forward relatively quiet during last week's 0-0 draw at San Siro without having to designate someone to shadow the three-time Ballon d'Or winner.
Consequently, Ambrosini sees no reason why they should change their approach in Barcelona.
"Messi is a unique player with the ball at his feet; one cannot think of trying to stop him," the veteran midfielder told reporters on the eve of the game.
"So, I do not think we will assign someone with the role of stopping him."
Ambrosini also argued that Milan cannot simply afford to invite Barcelona onto them for the duration of the 90 minutes, claiming that they will have to take the game to their hosts at some point if they are to progress to the semi-finals.
"If you leave them to play their game, sooner or later something happens," he pointed out.
"We must face them with courage otherwise this game will be very hard. It is great to be here but the intention is to go through."
Milan's meeting with Barca at Camp Nou gets underway at 20.45CET.
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