Though both require relative clauses to begin with a relative pronoun, Greek and English are typologically distinct in that one employs the pronoun with the gap strategy (English) and the other only needs the pronoun (Greek): a man who Chris saw [GAP] vs. τὸ ποτήριον ὃ ἐγὼ μέλλω πίνειν (the cup that I am about to drink [no gap]).
The English Gap is implicitly necessary for our processing of the grammatical relation of relative pronouns (in this case the Object relation), whereas in Greek, grammatical relations are marked by morphology* rather than structural relations/word order.
*This is not to say that English doesn’t have some morphological marking occurring on it’s pronouns, but this morphology is not the central marker of grammatical relations. Grammatical relations are centrally marked by structural position within the clause. Hence when English speakers hear relative clauses they consciously notice the Gap position where the Object would typically appear.