Doncaster set to land Man City defender Mills
The 20-year-old previously played under Rovers boss Sean O'Driscoll in a loan spell at Bournemouth when he was a Southampton player.
Mills also had a loan spell at Colchester last term.
"He is quick and strong in the air - he has got everything. It's just a case of putting it all together at the same time," O'Driscoll told Rovers' website.