"Tom and Courtney both have a chance of playing rugby by the end of the tournament," said Johnson.
"Tom is making good progress with the Leicester medics and our guys and it is the same with Courtney, so they are both week-to-week and hopefully they will be fitter sooner rather than later and make the end of the tournament."
Replacements for Lawes and Croft are due to be announced before the end of January, with former captain Steve Borthwick and Northampton flanker Tom Wood possible candidates.
Borthwick has not played for England since suffering a knee injury while captaining the side against Scotland in
the 15-15 draw in March.
Lawes came on as a replacement in that match against Scotland at Murrayfield and took the chance to impress in Borthwick's absence on the team's summer tour down under.
His rapid promotion to England's starting line-up followed a string of powerful ball-carrying showings for the Saints, including when he overshadowed second row pairing Paul O'Connell and Donncha O'Callaghan in their
31-27 win over Munster
in October 2009.
Lawes' club and international team-mate Ben Foden has tipped him to come back stronger than ever.
"He's a strong character and a young guy," said Foden. "Having had such a good autumn series, he'll be very disappointed not to be involved but he'll bounce back and hopefully the time in the gym will be spent well and he'll come back bigger and stronger than ever.
"He's got to get himself right first and foremost. He seems to be in quite good spirits. Injuries come and go in rugby."
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