Ok, this is just my take on it.
I think there is a massive difference between the 3CB lineup and the diamond we are now playing.
The 3CB formation gave the defence a very solid central core, with all three being, more or less in a straight line and within 10m of each other, with the wider CBs being able to cover the space behind the FBs, fully in the knowledge that the central CB was covering them and the middle.
Plus it had TWO defensive midfielders ahead of it, making the whole system simple to operate.
The new diamond formation gives us a defensive lineup that is effectively a W formation, (full of gaps if not operated correctly, or if there is a turnover in possession when the FBs have pushed upfield - see the move leading to the Sunderland penalty and the Man Utd first goal).
The Puel diamond sees the 2FBs push high up the pitch (on both sides, not just the side where the ball is), the 2CBs play further apart than they would in a normal 442 or 433, and the ONE defensive midfielder is supposed to sit slightly ahead of the CBs, but dropping in between them, should one of them get pulled wide.
There are so many things that can go wrong with this formation, when players are unsure of it, which is why I said yesterday (somewhere) that the key for me is how the CBs work as a unit with the defensive midfielder. Even if those 3 do work as a unit, the wider midfielders, need to fill any gaps through the middle, when the defensive midfielder drops into the defence, or when Romeu gets drawn out of position by chasing the man on the ball.
As I say above, just my take on it. It might work well, with the right person at the base of the diamond, and given time. For all the complaints we have had to come from behind twice, meaning that teams that are happy to defend deep are doing so. It would be good to see how effective the system could be, after taking the lead, forcing teams to defend less deeply and attack us.
My personal preference, given that I am an insecure football fan, who doesn't like change, would be 4231 or the 3CB option.
Great analysis, spot on. I'm with you - at the moment it feels like change just for change's sake. Our players fit nicely into the old system, and we are wasting points whilst trying to find our defensive balance.
For me, it's a bad sign that Puel persisted with only one DM away at Old Trafford.
That game was crying out for us to go and be difficult to break down.