The 26-year-old told BBC Sussex: "A lot of players we've got here have played at higher levels and won't be out of place playing Derby.
"We'll prepare the right way and treat it like we would if we were playing a team in this division."
He added: "We're going for the 'no fear' approach. We know the players they've got and the dangers but we've got good belief in our team."
The match has extra significance for the centre-back, who started his career with the Championship side.
Mills made 64 appearances over four years at Pride Park, before leaving to join Rotherham in 2006.
"Obviously I've got a love for the club, being my first club, but all that will be on the back-burner," he said.
"Subconsciously it may be there, but trying to win the game will be first and foremost in my mind."
Source: BBC Sport