He had to be replaced by Paul Pobga after 21 minutes of Sunday's 4-0 victory over Lazio in the Italian Super Cup following a rough tackle from Stefan Radu.
"Marchisio today underwent an MRI scan of the right knee that revealed a ... tear of his medial collateral ligaments," the club said on their website (www.juventus.com) on Monday.
"He is expected to be sidelined between 30 and 35 days."
The 27-year-old played a key role in Juve's back-to-back Serie A championship successes over the past two seasons.
The club start their title defence at home to Sampdoria on Sunday.
Marchisio looks set to miss Italy's World Cup qualifiers at home to Bulgaria on Sept. 6 and Czech Republic four days later.
Italy top their group with 14 points and four games to play, four ahead of Bulgaria and five in front of the Czechs.
Marchisio, who has won 39 caps, has featured in all six qualifiers played in this campaign.