Dee ihe na-aga n'ihu maka paragraf a - Cassidy bụ onye na-agbachitere na Ontario Hockey League (OHL) na Ottawa 67 si na 1982 ruo 1985, ndị Chicago Black Hawks nke National Hockey League (NHL) họpụtara ya na 1983 NHL Entry Draft
, họpụtara na agba nke mbụ, nke iri na asatọ n'ozuzu ya. Oge kachasị mma nke Cassidy na OHL bụ n'afọ 1982/83, mgbe o debere ihe mgbaru ọsọ 25 na 86 nyere aka maka isi 111. O meriri iko ncheta n'okpuru onye nchịkwa Brian Kilrea na 1984 na-enweta ihe 12. Mgbe ọ dị afọ iri na itoolu, o mere NHL mbụ ya na Black Hawks na Machị 1984. Site n'afọ 1984 ruo 1988, a wara ya ahụ ugboro atọ n'ikpere, gụnyere nnukwu mgbatị nke ACL ya.