The 26-year-old England full-back left Elland Road for Boro on a season-long loan deal last week following a fall-out with Reid in pre-season.
He was forced to sit out his new club's games against Arsenal and Leicester, but, after a delay caused by the Bank Holiday, clearance was finally given for him to make his debut this Saturday against Leeds.
The Teesside club insist there is no clause in the loan arrangement to prevent him lining up against Leeds.
A Riverside spokesman confirmed: "Danny Mills is available to play on Saturday. There is nothing in his contract and he is clear to play."
Mills, who was also a loan target for Birmingham, was not registered in time to play in the 4-0 defeat to Arsenal on Sunday.
He was in the stands again as Boro drew 0-0 with Leicester but will now go up against Stuart Parnaby and Robbie Stockdale for a first-team berth.