Chelsea midfielder Michael Essien has sustained another knee injury in preseason training with the Premier League club.

Chelsea says the 28-year-old Essien ''will undergo tests over the next few days to determine the extent of that injury.''

Essien was out for six months last year because of an injury to the same knee, ruling him out of Ghana's World Cup campaign.

He previously suffered an anterior cruciate injury in September 2008 that ruled him out until the following March.