Per Football Espana, coach Gerardo Martino had initially insisted that Pique missed the club's La Liga fixture at Osasuna due to fatigue rather than injury. However, his strain has been revealed.
The article states:
The defender missed last weekend’s league trip to Osasuna, with Tata Martino reporting it was due to fatigue more than any physical discomfort.
However, on from returning to action in the midweek draw away to Milan in the Champions League, the player has been confirmed by Barca as having now strained the hamstring muscle in his left leg.