Barker finished his playing career with the Millers and despite only joining up with Karl Robinson at Charlton in December he is set to leave the Addicks and team up with former team-mate Warne.
Speaking on the rumours earlier this month, Robinson said; "The man who has got the job is one of his good friends. It's a club Richie has played for and the fans look back on his time there as being very successful."
"You can see why there is speculation." he added.