Greenberg began his coaching career in 1977 as an assistant coach at his alma mater American University. From 1978 to 1984, he was an assistant coach at Saint Joseph's University under Jim Lynam. During Greenberg's time as assistant coach, Saint Joseph's appeared in the NIT in 1979, 1980, and 1984 and the NCAA Tournament in 1981 and 1982, including a run to the Elite Eight in 1981. Saint Joseph's also was the 1979–80 regular season champion in the East Coast Conference (ECC) and won the 1981 and 1982 ECC tournaments.