Ok so what is his justification for dropping Leao. He's frightened of leaving our CBs alone against a front two without a spare man, so that probably made him decide on a back three. He needs a 3 man midfield after seeing how stupidly easy teams are cutting through our midfield duo, especially now without Bennacer. So that means a 3 man midfield and a 3 man backline so only option is a 352 in this train of thought. There's no room for Leao in a 352. Origi (or Rebic) do a LOT more running off the ball than Leao, and since he can't drop Giroud, Leao is getting cut.
The other question mark is Messias. He's worried that Calabria won't give him enough threat and depth on the right, so my guess is he's asking Messias to run into that channel when we're on the break, so Calabria isn't forced to go too far forward leaving space behind, so Theo gives width on the left and Messias on the right.
The marking would be pretty simple and we won't have the usual problem against wingbacks that we suffer from, which is when to decide to hand over from winger to fullback. Theo is always on Darmian, Calabria always on Dimarco. Our back three keeps a man spare on Dzeko and Lautaro so we can always follow them when they drop and not worry about leaving acres of space. In midfield it's 3 on 3 and our two CFs just make sure they can't pass into their midfielders and have to keep pushing the ball wide. This isn't a bad idea in theory, but I still can't wrap my head around if it's worth Leao getting benched.
My hope is that they figured out that Inter massively drop their levels at around the 70th minute, and if we can throw on Leao, CDK, Rebic and a few other runners and go at them in the last portion of the game, maybe we can sneak a win. But in order for that to work we have to not be down a lot; and with the team confidence at an all time low, and with Gabbia and Tata on the pitch, that might be impossible.
It's a huge gamble by Pioli, and one that for sure will alienate almost everyone. I still can't believe he's done it. I really, really, really hope it works out and we win or at least not get embarrassed, because he deserves to stay on and fix this mess until the end of the season at least. But crazy shit like this, if it blows up on us, is going to really accelerate the end for him.