Buchan helped United win the 1977 FA Cup Final against Liverpool, which they won 2–1. This meant that Buchan became the only player to captain both Scottish and English FA Cup-winning sides. He also helped United reach another FA Cup final in 1979, where they lost 3–2 to Arsenal. He was still with United when they beat Brighton 4–0 in the replayed 1983 FA Cup Final, but he did not make the team for the final. Buchan left the club in August 1983 after injuries caught up with him, signing for Oldham Athletic on a free transfer. He had played in 456 games, scoring 4 goals, one of which was a superb, yet somewhat uncharacteristic, long-range drive against Everton.