Warnock has watched his team slide down the table following a six-match winless run and is desperate to bring in new faces to spark a revival.
"Johnson is a player I like and I will be speaking to him and his representatives," Warnock told the Croydon Advertiser.
Johnson is not a regular at the Riverside at the moment and has been loaned out already this season to Watford.
They, and several other Championship clubs, are also believed to be interested in signing the England U21 international.