"He will undergo tests over the next few days to determine the extent of that injury. Until we have the full results of those tests we will not be commenting further."
With Brazilian holding player Ramires to miss the start of pre-season training due to his involvment in the Copa America, losing Essien for any length of time would be a big blow for new boss Andre Villas-Boas as he seeks to regain the Premier League title from Manchester United.
Essien has been a key player for Chelsea since his reported £24 million move form Lyon in 2005, and won the supporters' player of the season award in 2007.
However, since that season he has only played 46 times in the league for the Blues as he has suffered several serious injuries.