It's key, in a 4-3-3 to have a guy to break from midfield to ensure there isn't a hug gap between the midfield and the forwards.
Lampard is probably the player who has done this best in recent years. It's a role that Marchisio could possibly have a go at, but there's no guarantee he could do it.
Without that player you need one of the forwards do drop off and get into a trequatista sort of position at times to stop a gap developing. Of the players Juve have been linked with, Sanchez is the guy who could start on the left and drift into that sort of area.
i think we should look more at is as a diffrent way
4-2-3-1 essentially is a 4-5-1, with 2 holding and 3 creative midfielders
you can look the same way at a 4-3-3, it also has 2 holding midfielders, it also has 3 creative players, and 1 real striker
the only diffrence is that 4-2-3-1 uses a trequartista, and 4-3-3 would use a deeplying playmaker.
is this really a problem ?
lets first look at our preferred formation
buffon
licht-bonuc-chiel-zieg
---inler-pirlo-melo
krasic------------bastos/sanchez/elia/heck even giovinco
--------aguero
when building up the play, there are plenty of options for pirlo to steer the play to, we would be able to stretch the play very effectively, while aguero can drop down and take part of the play aswel
when defending, both wingers should fall back and perform duties like a midfielder
most danger would come from counterattacks, having two very fast paced wingers, inler, and aguero.
this is why i think they want inler, because his box to box capabilities, and because the faster your midfield is, the less likely a gap between midfield and attack is going to occur