England's opening game of the World Cup comes against Fiji at Twickenham on September 18, just under six months from now.
This comes as a very cruel blow to Croft who missed all of last season with a knee injury and suffered a broken neck in 2012.
"He has had a bit of nerve damage as well," said Cockerill.
"We hope it is as quick as possible but it's likely to be six months or so.
"We just have to support the player and get him back on the field. It has been hugely disappointing for him and the club."
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