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Derby County's main summer signing George Thorne has been ruled out for up to nine months with a cruciate knee ligament injury.

Thorne only joined last week but was hurt in Tuesday's pre-season friendly loss to Zenit St Petersburg in Austria.

The 21-year-old former West Brom player was a transfer target all summer after impressing while on loan last season.

Manager Steve McClaren told the club website: "It's a huge disappointment. We wish him a speedy recovery."

Chatham-born Thorne was out for eight months of action in 2013 with a similar knee problem, rupturing the anterior cruciate ligament in a 2-1 defeat by Everton in February.

He will have an operation on Thursday and could be sidelined until March next year.

McClaren added: "At the time it looked to be innocuous as there was no-one near George and we thought he had just jarred his knee.

"Everyone's thoughts are with George and there is no doubt he will come back from this."