The midfielder will not require surgery according to manager Slaven Bilic, but will miss their opening three games against Manchester United, Southampton and Newcastle.
"It is a big shame that Cheikhou got injured in one of our early training sessions and it was out of nothing," Bilic told the Daily Mirror.
"He did his knee and he is going to miss the first few games of the season. It was a freak injury and it is a blow for us."
"He doesn't need surgery and the injury is not a bad one, but he needs rehab now."