I'd say Allegri is most to blame and Marotta least. Marotta addressed two of three core issues, that is an aging squad and replacing those outgoing core players with quality additions. He never managed to get the trequartista that Allegri wanted.
Allegri is just a bit too liberal a manager in my view. It was the right approach with a ready made squad full of experienced, certified champions who'd had one or two seasons winning together. He left players like Tevez and Pirlo to their own devices to find their best football by themselves. The slate has been wiped more or less clean now and it's been up to Allegri to instill his tactical ideas and discipline in a virtually new squad and he has thus far failed in doing so. I guess that's just not one of his strengths. Kind of like the anti-Conte.
Your forgetting the many disjointed mercato decisions along the way. From letting Coman go, to then getting Cuadrado. The fiasco with Vidal, the Draxler back and forth midway through July. The Alex Sandro U-turn after nearly signing that other guy from Athletico. Hernanes on the last day of the mercato. The position that ironically had to be filled before any other position, being AM supposedly/notionally came in the form of Hernanes on the 31st of August, by which time pre-season was done and dusted and Allegri had to find multiple tactical and technical solutions to make his preferred formation work. On paper, we can look at the likes of Mandzukic, Rugani, Dybala, Cuadrado, Sandro etc. and think thats great, but our starting team was weaker, and only our depth was bettered. Sure, the injuries to other starting key players didn't help, but spending 18m on Zaza when there were clearly other pressing needs shows that the management and the coach were not in tandem.
Big clubs don't function like this in the mercato. We worked without any plan whatsoever after selling Vidal, by which time things were precariously placed because Tevez and had anyway already informed all and sundry that he wanted to move on. Two major pillars of the team gone, and we had absolutely nothing to show for it at the end of that. I don't care about having a flashy mercato, but i do care about having an effective one. And all things considered, we've only achieved soft objectives on this mercato which has been one of the most directionless in all my time of having watched and supported this team and club. Absolutely pathetic.
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By what logic should this squad provided warrant a 12th place? If you're blaming Marotta, you're blaming the perceived lack of quality of the players he bought but they're not players worthy of a mid-table side at all.
Being 12th place right now doesn't necessarily mean we'll finish there. We may finish 6th or 7th, but the fact of the matter is, that we didn't prioritise the true needs of the team this summer. We prioritised opportunity. A formula thats not sustainable for success at the highest level.
This is not to defect blame from Allegri's decision making, but to say Marrotta has little fault in all of this, is borderline blind.