Mezue ahịrịokwu na-esonụ: Cassidy bụ onye na-agbachitere na-egwuri egwu 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, ahọpụtara na agba nke mbụ, nke iri na asatọ n'ozuzu. 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.