Michael Essien could be set for another lengthy period on the sidelines © Getty Images Enlarge

Chelsea face a nervous wait as midfielder Michael Essien is feared to have seriously damaged his knee at their Cobham training complex.

Essien will be viewed as a key cog in the Blues' attempts to wrestle the Premier League title back out of Manchester United's grasp, but the club confirmed on Friday that he could miss the start of the season.

"Michael Essien suffered an injury to his right knee during training this week," said a club statement. "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."

The news was first broken on Twitter by former Chelsea player Jody Morris. "Sad news about Michael Essien... first day full training as well," he tweeted. "Sorry tweeps...heard he done his knee..cruciate."

Essien missed much of the 2008-09 season with a damaged cruciate knee ligament before succumbing to a medial ligament injury at the African Nations Cup in 2010, missing half the 2009-10 campaign and the World Cup.

Should the worst prove to be true, then the Andre Villas-Boas regime would be expected to enter the transfer market for a player in a position where a pool of Ramires, John Obi Mikel and youngster Josh McEachran already looks stretched.

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