Czech Republic international Cech, 29, fell awkwardly at the end of training on Wednesday and will now miss Saturday's Premier League clash with West Brom and the following weekend's visit of Norwich.
Blues boss Andre Villas-Boas said: "Petr Cech yesterday suffered a medial ligament injury and will be out for three to four weeks."
He added: "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 injury looks to have also ruled Cech out of his country's Euro 2012 qualifier against Scotland at Hampden on September 3.
Cech, who kept a clean sheet in his team's 0-0 Premier League opener at Stoke last weekend, could be back in contention for the September 10 trip to Sunderland.
He has been a commanding figure since arriving at Stamford Bridge from Rennes in 2004, helping the London club win three Premier League titles, three FA Cups and two League Cups.
Portuguese stopper Hilario, 35, looks set to take over between the posts for the Blues in Cech's absence, although Villas-Boas could turn to 26-year-old former Middlesbrough keeper Turnbull.
Teenage goalkeeper Thibaud Courtois joined Atletico Madrid on a season-long loan straight after signing for Chelsea from Belgian from Genk last month.
Source: Team Talk
Source: Team Talk