Chelsea goalkeeper Petr Cech has been ruled out for three to four weeks after the 29-year-old suffered a medial ligament injury in training.
The Czech Republic international claimed 15 clean sheets in the Barclays Premier League last season, and began the season with a shut out against Stoke City on Sunday.
However, Cech's latest injury will keep him out for this weekend's visit of West Bromwich Albion, and then of Norwich City a week later.
Chelsea manager Andre Villas-Boas told reporters: "It was the end of training, a normal training incident. He fell awkwardly with his weight on top of him. We expected much worse when we saw the incident."
The stopper is set to also miss the trip to Sunderland, while he's doubtful to face Manchester United on September 18th or feature in Chelsea's first game in their season's Champions League group phase.
Cesh will also be considered a doubt for Czech Republic's Euro 2012 qualifier against Scotland, with the Czech's lying second in Group I behind unbeaten Spain.
The Blues made the signing of Thubaut Courtois earlier this summer, but with the Belgian currently on loan with Athletico Madrid, Villas-Boas will have to choose between Hilario or Ross Turnbull, whose appearances for the club have been limited, such has been Cech's dominance between the sticks at Stamford Bridge.
Cech's injury is the second serious knee injury to befall Chelsea, following Michael Essien's cruciate ligament problem, which will see him miss six months of action.