The 22-year-old defender enjoyed a fruitful season with Doncaster, winning the club's player of the year award following their successful bid to avoid an instant return to League One.
Their success has seen a number of their players linked with moves away from the club, with Ryan already moving to reject an offer from Leicester for Ritchie Wellens.
And with Birmingham having already asked about the availability of Mills, Ryan is bracing himself for further bids.
"Our players are highly thought of," he told the Star. "I wouldn't be surprised if Birmingham came back in for Mills, but at this stage we've not heard anything.
"But it is early days and I don't think that anything much will happen until after the play-offs are over and players have been on holiday.
"There will be a lot of speculation in the media but a lot of it will just be that."