The Leicester Tigers flanker damaged the medial ligament of his left knee after 27 minutes of his club's crushing Heineken Cup win over Viadana at Welford Road on Saturday.
Croft, who won a pre-tour call up for the Lions following Alan Quinlan's suspension, is expected to be out of action for five weeks.
That length of absence rules the 24-year-old out of England's Six Nations opener at home to Wales on February 6, as well as the trip to Italy a week later.
If he recovers on time, Croft could return to the England fold for the match against Ireland on February 26, although his only chance of action beforehand would be Leicester's Guinness Premiership clash with Gloucester on February 20.
A winner of three Test caps on the Lions' summer tour of South Africa, Croft will also miss Leicester's crucial Heineken Cp pool decider at the Ospreys next weekend.
The former Oakham schoolboy had only just returned to action following a month-and-a-half on the sidelines having picked up an earlier injury to the same knee at the beginning of December.
"It's a blow to injure the same knee," said Leicester head coach Richard Cockerill.
"England manager Martin Johnson might be concerned about Tom's Six Nations situation, my only concern is that we won't have him in Swansea next weekend.
"I have my job to do, Martin has his. But Crofty is a world-class player who hadn't seen action for ages. He needed playing time, so I had no qualms starting him."