The 29-year-old underwent a scan at Chelsea's Cobham training base after limping out of Tuesday's Champions League group game at Steaua Bucharest and the extent of the damage has now been revealed.
A statement published on Chelsea's official website read: "Chelsea Football Club can confirm Fernando Torres suffered a medial collateral ligament injury to his left knee in the Champions League group game at Steaua Bucharest on Tuesday evening.
"The 29-year-old will be out for approximately three weeks."
Blues manager Jose Mourinho had expressed his hope after the 4-0 win in Romania that Torres would be fit for the Premier League contest with Cardiff on October 19.
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