Born in Moita, Setúbal District, Diamantino was one of S.L. Benfica's key players during the 1980s, winning several Primeira Liga and Taça de Portugal titles. He also played in the UEFA Cup final in 1983, lost to R.S.C. Anderlecht (0–1 and 1–1), missing the European Cup final in 1988 due to an injury, in another defeat, this time to PSV Eindhoven on penalties.