We lose the midfield every week. It puts added pressure on our back line and we can’t link back to front. Realistically we need two starting caliber mids. CB can wait in comparison
It's true. But the question is, everyone on the pitch knows the defense is not reliable, even the CBs knew too. If two low confidence and mediocre CB get panic in a 4 man back line. You could potentially find yourself leaking 2-3 goals within 10 mins because when you will play more up the field with far more possession, will undoubtedly leaving far more space for opposite team's forwards to run for.
I guessed what changed the approach is the result of Villa away (0:3) and Watford home (2:5). Conceding 3 goals and 4 goals within like 10 mins or so. With the obsession of defensive as core, it's like slapping the manager's face. Why in a sudden a toothless Watford can find 5 times in the net? All those goals happened when you got more men in midfield but being counter attacked. And the CBs were like traffic cones and certainly won't help. And the full backs (Digne and Coleman) were slow and out of position to defend these by that time too.
He's so afraid of the defense (mainly CBs) being exposed, so to sacrifice the midfield and possession as the lately 541 reflected. That's a logical guess with someone like him, being in the football arena for more than 40 years or so. It's like a gambling to play 4231 or 433 at this moment in his mind, betting opposite team won't be able to exploit the defense. Then you can win with possession, fluidity and comfortably like start of the season (WITH Mina though). Set pieces? Not enough players with good heading clearance abilities, low confidence, out of position, so many could go wrong.
Certainly those losses of Villa and Watford taunted him. Unless he felt more confident of his own defense, he won't change the 541 formation. He's always rational and statistical, against gambling. All things combined, end up the situation like this. Otherwise, unless he's got Alzheimer, why he didn't play his most favoured formation in his life (4231/433) at a moment while everybody is screaming for.