The 29-year-old will miss the Blues' match against West Bromwich Albion on Saturday along with the clash against newly-promoted Norwich City on Saturday, August 27.
"Petr Cech yesterday suffered a medial ligament injury and will be out for three to four weeks,” Chelsea manager Andre Villas-Boas said.
"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."
Meanwhile, David Luiz remains sidelined through injury but is expected to make his return against Norwich next weekend.