Chelsea midfielder Michael Essien has sustained a knee injury in training.
A statement from the club said: "Michael suffered an injury to his right knee during training this week.
"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."
Essien also missed the 2010 World Cup because of a knee problem, and the midfielder's latest setback is a blow to new Chelsea manager Andre Villas-Boas, who is preparing his squad for their trip to the Far East, where they will compete in the Asia Cup.
ESSIEN'S KNEE TROUBLES
Continue reading the main story
- September 2008 - misses six months following injury while on Ghana duty
- January 2010 - ruled out of World Cup after breaking down at Africa Cup of Nations
- July 2011 - injured in training and set to undergo further tests
Essien joined the Blues from French club Lyon in 2005 in a £24.4m deal and has gone to to establish himself as a key player at Stamford Bridge.
He has helped Chelsea to two Premier League titles and three FA Cups, as well as the Carling Cup in 2007.
Despite being ruled out for the majority of the 2008-09 campaign, Essien has amassed 229 appearances for the London side, scoring 26 goals.
While the club have not elaborated on the severity of the problem, Essien will be mindful of what happened to him while playing for Ghana in a World Cup qualifier against Libya three years ago.
On that occasion he landed awkwardly during the match and was subsequently told he faced a lengthy spell on the sidelines.
Meanwhile, the Blues have named Michael Emenalo as their new sporting director.
Emenalo was assistant first-team coach for most of last season, taking on the role after Ray Wilkins left the club.
The 45-year-old will oversee a scouting network and youth development programmes, as well as supporting Villas-Boas.
A statement on
Chelsea's website read: "Michael takes on a vital role that will assist the overall long-term football strategy of the club."