Aston Villa manager Steve Bruce says Jack Grealish's injury is a massive blow to the club just days before the start of the new season.
The midfielder was forced off during his side's pre-season friendly against Watford at Villa Park on Saturday after damaging one of his kidneys.
The game was a tribute to Graham Taylor, who died in January.
The former England manager had successful spells in charge of both clubs.
Grealish scored five goals in 31 appearances for the club last season including a goal against Brighton on the final day.