Without a question, buying Diego was the biggest mistake his club did. Not because Diego is a bad player or anything, but because we built the entire team around this guy. Our entire tactical setup depended on Diego. He was supposed to be our new savior, and I think it's that remarkable expectation that was set by the board that really fucked things up for us.
You could be right here. Tough to pick the biggest mistake, but in hindsight this really fucked us up.
Ranieri made too many mistakes and it wasn't easy to trust him, but he still knew our team very well and he knew that a transition from 4-4-2 (the game plan we were adopting for years, with players used to it) to something like 4-3-1-2 without wing play should have been done gradually and not try to do it immediately.
We wanted to do it by buying one single player. Even if that player was much better than Diego, we were still going to suffer a lot because we didn't have the full backs and we didn't have other players who could play those tactics.
The result is known. 7th placed last year and a huge set back in the following years.
Diego as a player might be good, and he is indeed very talented, but the decision to buy him and set the club around him, without improving some other vital positions for 4-3-1-2, turned out disastrous.
P.S: The decision to immediately sell him is also something I still can't get over. We wanted a 4-3-1-2 with Diego, we bought Diego, then the logical move was to improve other positions instead of going back to 4-4-2, selling Diego and changing almost everything.
Everything about Diego turned out to be a disaster.