Croft has undergone knee surgery after a blow to the joint in Sunday’s Aviva Premiership opener against Worcester left him with a ruptured anterior cruciate ligament.
The 27-year-old only returned from a career-threatening neck injury nine months ago and went on to play for England at the Six Nations and for the British and Irish Lions on their tour of Australia.
“It’s horrendous bad luck for Tom because it is only last year that he came back from a very bad neck injury and did so well for England, and the Lions as well,” said Cockerill.
“But these things happen. Tom will make a full recovery, but it’s his season over. It is a blow, but we have just got to get on with things.”
Croft’s absence will be a major blow to England head coach Stuart Lancaster as he prepares for the autumn internationals and next year’s Six Nations.
The forward, who has battled back from two previous knee injuries, returned from injury in January and fought his way back into the England team to start the final Six Nations match in Wales and then earned a place in Warren Gatland’s Lions squad to play Australia.
“He is in good hands with the Leicester medical team and our own medics will be in regular contact to help in his recovery,” said Lancaster.
“We wish him all the best.”