England are set to name an injury replacement for Moody in their squad next Monday.
Contenders are likely to include Tom Wood, Chris Robshaw, Andy Saull and Phil Dowson, a quartet who were selected last week in the England Saxons squad.
Johnson said: "It is always disappointing to lose a player and leader of Lewis' calibre.
"But as I have consistently said. good teams adapt to injuries, and that is what we intend to do.
"While this could still mean Lewis plays a role in the RBS 6 Nations, it does mean that another player gets the chance to step up and make it hard for him to get the shirt back."
Johnson will announce his captain in the week leading up to the Wales game, with Moody now remaining at Bath for his rehabilitation in close consultation with the England medical team.
He could though, be available for England's final two Six Nations games against Scotland and Ireland in March.
With Moody and Croft now absent, Johnson's back-row combination will
now take on an unexpected look against Wales when England launch their quest for a first Six Nations title since 2003.
Leeds Carnegie flanker Hendre Fourie would be a strong favourite to wear the number seven shirt, with either Joe Worsley or James Haskell the probable blindside flanker options.
Wales have also been hit by problems ahead of the tournament opener. Prop Gethin Jenkins is out of the whole competition because of a toe injury and scrum-half Richie Rees is banned until March 31.
Wales boss Warren Gatland has additionally seen Gavin Henson's Test comeback hopes put on hold by a calf muscle problem, and Gatland is anxiously awaiting news on tighthead prop Adam Jones, who suffered an elbow injury during Ospreys' Heineken Cup defeat against London Irish two days ago.
Jones is expected to undergo a scan today, amid initial unconfirmed reports that he could be out for up to eight weeks.