It's a tough question and a reason to keep hold of Koeman to see what's available in the future. Unsworth would undoubtedly produce and initial spike in results but it remains to be seen if he could actually manage fully fledged established players.
I've began to think there's something a bit rotten at the core of Everton the way we keep producing these diabolical performances - tactics aside the players just don't seem assed at all.
If we were to replace anyone my realistic choice would be Tuchel or perhaps Nagelsmann.
Someone young with fresh ideas and a fresh approach.