This is the crucial thing for me.
Yesterday, and against Spurs and in a few other games recently, I can hear myself saying "Second balls, City. Second balls!" over and over. Almost all of them are bouncing to opposition players. We're losing 50/50s constantly. It's all coming from Walker, Gundogan, and Bernardo (and, to a lesser extent, De Bruyne) just not having the stamina to chase and fight anymore. Throw an exhausted Gvardiol and Lewis into the equation and I think it's easy to see where the problems are coming from.
We usually beat teams by grinding them down, tiring them out, making sure they're doing all the running and chasing. We get to the ball first, we win it back quickly, we win it back high up the pitch. Sooner or later, the opposition tire out and gaps form, and that's where we strike. But this season we're losing the ball because Walker, Gundogan, and Bernardo have all lost a yard, so too has De Bruyne. It all adds up, hence a knackered Haaland and Foden not pressuring Martinez before Amad's winner last night.