The Leicester Tigers player suffered medial knee ligament damage in his club's 47-8 Heineken Cup win over Viadana on Saturday. He will therefore miss the clashes against Wales and Italy, and his participation against Ireland at Twickenham on 27 February is also in doubt.
A statement from the Rugby Football Union read: "A scan has confirmed Croft sustained a grade two medial cruciate ligament injury to his left knee in the Heineken Cup game against Viadana. He has now started his rehabilitation programme with a view to returning to action in five to six weeks."
Croft established himself firmly in the international line-up during last year's Six Nations, where he was used as a substitute in the first three games and impressed sufficiently to earn a starting spot for the final two matches against France and Scotland. He has since become a key figure in England's plans and earned a place on the British & Irish Lions' summer tour of South Africa as a late call-up after Munster's Alan Quinlan was ruled out by suspension.
He made an immediate impact with a try in the first tour match against the Golden Lions and added two more in the first Test against the Springboks.
Northampton's Courtney Lawes will be one of the men in contention to replace Croft.