I'm willing to give Mangala a chance if Pep thinks he can do it, but last season for example, the back 3 system allowed teams to run at us with pace & our cbs were unable to do anything once a pass, even a short pass, was made to people such as Vardy. They didn't read the pass nor anticipate the danger, in the situation before the pass & once the pass was made, they were two, three yards too slow to close down the recipient, it's then just upto the keeper, or the oppo to fall over/miss the target, or we'd had it.
Chelsea's centre backs were much more proactive & often snuffed it out or fouled, before the situation even developed. We can't see it coming & we were absolutely hopeless at seeing it v Real Madrid & when we did see it, too late, we didn't have the pace to recover & just relied on hope, which worked.
As usual, the defensive shortcomings are being ignored on here & everyone was 'fantastic' at the back defending. But they weren't, non of them.
Acceleration over 10-20 yards, rather than a 50 yard sprinter, might be enough to give us that split second which makes the difference, as we saw with the fullbacks last night.