the threat from the Barca front 3 will force Pirlo & co to play more defensively which will suit Barca - they will boss the centre of the pitch which, naturally, will cut the supply line to the Juve's strikeforce, nullifying their attacking threat.
one mistake Juve will make is to entrust Pogba on Messi...that'll be suicidal but am sure they won't do that! on his current form, Messi needs to be manned by a defensive personality or rather a collection of defenders. we all saw what he recently did to Boateng (Bayern) plus the way he singlehandedly destroyed the entire "village of defenders" vs Bilbao last Saturday.
one more thing...don't forget Barca's threat doesn't originate from Messi alone. you have a Neymar. an Iniesta. needless to mention the dracula (Suarez) himself!