I would say Ivey was just a symtpom. We really only had maybe 3 good defenders in the rotation last year, EHJ/Gillis/Morton. All fo this is just based on my own (flawed I'm sure) understanding of basketball.
Furst was pretty poor with rotations but he was just a freshman - nothing out of the ordinary really.
Sasha was not a good defender, he didn't kill us defensively but wasn't much of a help either.
Tre was Tre, he could do some awesome things defending the PnR with his insane wingspan but other than that was generally poor as a defender.
Ivey wasn't here to play defense (TIC), he was pretty good on-ball but didn't always focus defensively off-ball.
Edey is an average defender, not the best rim protector but he is still 7'4" and struggles away from the hoop. He is better than Haas was.