Miller joined Manchester United for free on 1 July 2004, at the expiration of his Celtic contract. He made his United debut on 11 August as a 67th-minute substitute for Darren Fletcher in a 2–1 win at Dinamo Bucharest in the first leg of the third qualifying round of the Champions League. Four days later he made his debut in England's top flight, starting in a 1–0 loss at eventual champions Chelsea on the opening day. He scored his first goal for the club on 26 October against Crewe Alexandra in the third round of the League Cup, finishing a pass by David Bellion in a 3–0 win at Gresty Road. After the start of his career for the Red Devils, he found first-team opportunities rare towards the end of the 2004–05 season. He was given a rare first-team start in January 2005 in an FA Cup tie against non-league Exeter City, but turned in a poor performance and was substituted in the second half. In his entire time at the club he made only 22 first-team appearances. Miller later commented that "Man Utd didn't work out but I have no regrets about giving it a go."