And the returning Tony Barras, who was suspended last week, is struggling with an ankle injury, meaning Nicky Fenton could be the only fit centre back.
Mills, talking about his team's discipline told the Evening Post: "It is something I am going to clamp down on. We can't be missing players for no reason.
"I believe in discipline and if players step out of line, they will soon know about it.
"I don't mind if they are legitimately booked after going in for a tackle but it is the daft stuff I want to cut out.
"We don't want to be picking up bookings we don't need to.
"I thought Richo was superb against Wrexham. He and Nicky Fenton both were.
"They defended like a pair in the heart of defence and I thought they were both fantastic.
"Ideally, I would like to have kept the same team, apart from Mark Stallard who's gone to Barnsley. More than anything you want to keep your back four together."
Elsewhere, regarding the on/off transfer of Tommy Johnson, Mills said: "We have had a chat with Tommy and he wants to come, that situation has not changed.
"Things are looking positive. All that remains is for things to be sorted out at the Gillingham end. We are just waiting for that to happen.
"But obviously he needs to have signed today if he is going to play tomorrow and time is running short."
Johnson has now been named in the Gillingham squad for their game tomorrow so will definetly not feature for The Magpies tomorrow.