Manchester United have the weakest defensive record in the top half having conceded 25 goals in 15 games this season.
Throughout the whole Premier League only five teams have shipped more goals than United and Mills thinks De Gea is partly to blame.
It comes amid Jose Mourinho continuing to struggle to find a winning formula at Old Trafford.
“When he first came in there was progression, we saw that. He won a couple of trophies and finished second in the Premier League,” Mills said of the United boss.
“Now, for some strange reason, they have regressed quite badly.
“You’ve got one of the best goalkeepers in the world who is making some strange, bizarre mistakes.
“You’ve got a defence that was one of the best defences in the league is now one of the leakiest around, certainly in the top half of the table.
“It’s a very similar group of players, not a lot has changed among that group of players, but it seems to have gone backwards quite dramatically.”
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Mourinho identified his backline as an area they needed to strengthen in but executive vice-chairman Ed Woodward failed to bring in his top targets in the summer.
“It’s very strange that the club didn’t back him,” Mills said on the Sky Sports The Debate show.
“They gave him a long term contract but didn’t give him the funds to invest.
“Okay, he might have made mistakes in the transfer market, but what manager hasn’t done that?
“He clearly wanted some centre-halves, he wanted to bolster his defence so he could play however he wanted to play. That didn’t happen.
“The defence changes week in, week out, and they are conceding goals for fun.”