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Key battle: Lucas Moura's counter vs. whoever's there to stop him
Barcelona's resurgent performance against AC Milan at the Camp Nou will have sent a chilling sensation down the other seven sides' spines.
When they're in dominant form, there's just no stopping them.
Paris Saint-Germain will feel they have the tools to pull off a similar game plan to the Rossoneri, only this time to make it work over two legs.
The flow of play is very predictable. Barca maintain possession and probe, and on occasion, PSG counter with pace and try to expose the flanks.
Milan had a frightened Stephan El Shaarawy and a young M'Baye Niang to lean on, but the French side have arguably better options.
Lucas Moura is a man embracing his price tag and performing to the highest level, and his trickery and pace will be an issue for Barca's defence.
Blaise Matuidi is a key man in midfield and is 10 times the athlete the aging Massimo Ambrosini remains.
In a way, the key battle should be Lucas Moura vs. stage fright. After all, if he can overcome the occasion, he can be the key to Carlo Ancelotti's side winning.