The former Real Madrid midfielder came on as a late substitute against Shakhtar Donetsk in the Champions League on Tuesday only to be stretchered off in the last minute.
The 26-year-old was introduced as an 81st-minute substitute in the 2-0 Champions League defeat to Shakhtar Donetsk on Tuesday, but had to be stretchered off after twisting his right knee as he chased down Fred in the final minute of normal time.
Head coach Jagoba Arrasate expressed his concern over the injury after the match and his fears have been confirmed, with Granero having suffered an anterior cruciate ligament (ACL) injury.
QPR, his parent club, Tweeted on Wednesday: "Best wishes for a speedy recovery to @eGranero11 following confirmation that he suffered an ACL injury last night".
Two goals from Alex Teixeira consigned la Real to defeat in their opening game in the competition this season.