The 31-year-old will miss the Champions League third qualifying round against Villarreal as well as the start of the Premiership season.
The former Derby player had recovered following knee surgery but was injured again during a pre-season friendly.
Thc club physio said: "We don't know how long he'll be out for exactly."
Phil Neville, who yesterday joined the club from Manchester United is expected to take Carsley's place in midfield.