I honestly think the player we've been missing the most is Evans. I've said before how well he organises the defence, we're now seeing that without him Soyuncu is, at best, utterly unremarkable, Vestergaard hasn't, at best, settled in, and out great full backs haven't looked anything of the sort. Brendan didn't come with any reputation for defence coaching that I'm aware of, and I wonder if Evans' presence and ability has been what was papering over this enormous crack.
With the lack of alternative leaders in the squad, I don't see what can be done until the transfer window reopens irrespective of what happens to the manager. I'd be surprised if Evans doesn't end up retiring over the summer, so an organiser at the back will be absolutely critical.