The 21-year-old winger initially joined the U’s on loan from Everton in July 2023, however a serious knee injury suffered last January saw Mills ruled out of action for a lengthy period of time.
He returned to first team training with the Toffees in December and then joined United on a permanent basis at the end of January.
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The U’s start their Sky Bet Championship campaign at home against Portsmouth on Saturday, and Mills is hoping to kick on following an encouraging pre-season.
He said: “It’s my first pre-season that I’ve been able to do for a while, and to just get in the rhythm of playing games and training constantly has been good.
“I think it’s been good. The performances have been building slowly and looking forward to Portsmouth has been the end goal.
“We’ll see who the gaffer decides to pick. There’s plenty of competition in the team for everyone. You’re never guaranteed a starting place, but I think I’ve done what I can to maybe try and be in that team come Saturday.
“I feel fit. I feel ready. That’s all I can ask for really. I’m excited for the weekend.
“One of my main aims is to try and stay as fit as I can for as long as I can. That’s a big one this year.”
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Mills added: “We feel ready. It’s a really good group here. We’re going into the weekend ready, and we’ll attack it with everything we’ve got.
“I think when we all come together, the outside noise doesn’t matter to us. We know what we’ve got in that changing room and we’ll work together to give it our very best.”
United have added five players so far during the summer transfer window, and it is right back Brodie Spencer who Mills has formed a promising connection with during pre-season.
Spencer joined the U’s from League One side Huddersfield Town for an undisclosed fee, with he and Mills enjoying some good link-up play down the right-hand side in pre-season for United.
Mills said: “I’ve enjoyed playing with Brodie so far. I think our games complement each other quite well. I’ve enjoyed it.
“The boys that have come in have been excellent in training and in games. I think they’ve really showed what they can do. Hopefully they can keep that going throughout the season.”