Wales international Ledley, 31, has not appeared for Derby since he sustained a back injury in the 0-0 draw at Hull City on Boxing Day.
Fellow midfielder Johnson, 30, has not featured since the 1-1 draw at home to Sheffield United on New Year’s Day after fracturing a bone in his back.
Ledley has resumed training and is set to return to the squad for Tuesday’s Sky Bet Championship fixture away to Millwall, while Johnson was due to train on Monday morning for the first time since being sidelined.
Rowett told RamsTV: “Joe is fit and will be involved in the squad for the Millwall game.
“Bradley is due to train today and he has made good progress. I think it was felt that the fractures he sustained in his back won’t get worse by playing; so it’s just a case of managing the pain.
“After a period of rest he will train and we’ll see how he is. That’s a positive for us.”
Second-choice goalkeeper Jonathan Mitchell will miss the trip to Millwall after injuring his finger during last week’s warm-weather training camp in Dubai.
Dutchman Kelle Roos, who recently returned from a loan spell at League 1 Plymouth Argyle, will act as Soctt Carson’s understudy.
Rowett said: “Jonny injured a finger out in Dubai and we hope it isn’t too serious.
“Kelle Roos will come in on the bench and has proved from his time out on loan that he is more than capable of filling that role.”