He made his debut for United on 20 February 1988, scoring in a 1–0 home win against Barnsley. Despite this promising start the team was in decline and the club was relegated at the end of the season. The following season proved a different matter as Agana struck up a striking partnership with new signing Brian Deane. During the 1989–90 season he scored 24 goals in 46 appearances, which, along with a similar tally from strike partner Deane, helped United win promotion from the Second Division at the first time of asking, with Agana being named fans' player of the year. The following season saw more of the same, with United promoted again to take them into Division One. That was thanks in part once more to Agana's goals, which included a brace in the 5–2 victory at Leicester City that clinched promotion.