"Unless that gap gets closed in some way it's unlikely that will take place.
"We need another centre-half and have been working hard and will continue to do that."
Zak Whitbread of Millwall is now being linked with the Reds but Mills may not be out of the equation just yet.
Doncaster number two Richard O'Kelly was in Europe at the weekend, spying on a potential centre-half signing for his club.
"It was a very competitive game and the lad I went to watch did okay," said O'Kelly, quoted in the Sheffield Star.
"We are sort of persevering with that. We need to speak to his agent and a few things need to happen."