The Championship club is experiencing a financial crisis while it waits for a takeover by Sheikh Khaled's Derventio Holdings to go through.
A deal to sell the club was agreed by owner Mel Morris in November, with a transfer embargo currently in place.
Morris is said to be reluctant to inject his own cash into the club ahead of the takeover.
The Daily Mail says Rooney has stepped in to help the Rams out.
On a two-and-a-half-year contract, he has told the club that he will defer part of his salary to divert funds to his players.
The squad's December wages were not paid on time but Derby reportedly reimbursed players on Monday and have stated that January's wages will go through on Friday as planned.