The 21-year-old has joined Carlisle on a six-month loan.
Rotherham have been busy trying to sign a striker this summer.
The Millers boss met with the media on Friday.
The former Sunderland and West Brom defender has joined on a two-year deal.
The Millers defender is heading back to Aberdeen.
The Millers have made three signings so far.
The Championship clash will be played on Sunday August 26.
Marek Rodak, Josh Emmanuel and Richie Towell were key players last season.
The Millers play two friendlies in Austria.