Coach Antonio Conte admitted to fears over Pirlo on Sunday after the experienced midfielder hobbled off 15 minutes into a 1-0 win over Udinese in Turin.
Pirlo left the pitch for treatment and despite returning to the pitch to play on, pain got the better of the Italy international.
A Juventus statement said: "After departing yesterday's Juventus v Udinese match early in the first half, Andrea Pirlo underwent medical tests this afternoon which revealed second-grade damage to the medial collateral ligament of his right knee."
The Serie A champions did not say for how long Pirlo would be ruled out but such injuries normally require a one to two-month recovery period.
An injury-time header from Spain striker Fernando Llorente rescued Juventus in Turin on Sunday and sent them three points clear of Roma at the top of Serie A.
Juventus travel to Istanbul in ten days time where they require a draw against Turkish giants Galatasaray to qualify for the last 16 of the UEFA Champions League.