Juventus said in a statement that an MRI had revealed a sprain and that "30 days will be necessary for a complete recovery".
The announcement is an early blow for new coach Massimiliano Allegri, who took over in July after Antonio Conte resigned.
Conte had led Juventus to three successive Serie A titles in as many seasons with Pirlo as the team's main inspiration.
Juventus are away to Chievo in Serie A's opening match on Saturday (5.00 p.m. BST).
(Writing by Brian Homewood in Berne, editing by Stephen Wood)
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