The 29-year-old is certain to miss Sunday's Premier League trip to Aston Villa and a major question mark hangs over whether he will be fit for England's crucial Euro 2008 double-header.
McClaren's men are up against Israel at Wembley on 8th September before taking on Russia four days later.
A statement on Chelsea's official website read: "Chelsea can confirm that Frank Lampard was injured in training and a scan has revealed that he has a tear in his left thigh.
"It is too early to say how long the injury will keep him from playing."
