The former Vatreni international defender believes that the new-look Italians pose a big attacking threat which can only be stemmed by closely marking their playmaker
Prandelli's men produced an impressive performance in their Group C opening match - a 1-1 draw against reigning champions Spain.
Croatia currently top the group after their 3-1 victory over Republic of Ireland, but Jarni believes Italy will present his compatriots with a much stiffer test when the two countries clash in the Stadion Miejski in Poznan on Thursday.
Talking to 24Sata, Jarni highlighted Juventus playmaker - and 2006 World Cup winner - Pirlo as the Azzurri's heartbeat and supply line.
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"Prandelli has completely changed Italy, they score more goals now and are no longer considered defensive," he stated.
"Italy will probably play 3-5-2 and look to push forward but, due to our disciplined formation, they will not have as much space as they did against Spain.
"Regardless of the likes of Balotelli, Cassano and Di Natale in attack, the key to Italy's game is in their midfield. Marchisio is doing a phenomenal job and even more attention needs to be paid to Pirlo.
"We must not allow him too much time on the ball. We will play against a team who have been world champions, a team who cannot be compared with Ireland. But no matter what, I think we can beat Italy."