Ideally you want at least 1 good dynamic scorer infront of him. Doesn't necessarily need to be a CF, it could be a scoring winger with the skills and capability to slot in the middle during games and drags CBs off Messi.
You can still keep the 4-3-3 formation with Messi in the middle, but get two wing-forwards who are more than capable of being CF, like the Eto'o/Henry partnership was.
With Eto'o/Henry you had:
Henry----Messi----Eto'o formation that could quickly change into:
Henry----Eto'o-----Messi or:
----Henry---Eto'o-----
-------Messi--------
That's the flexibility needed.
With Neymar now taking the role of Henry (he still needs to mature into that role), you just need an Eto'o type of dynamic forward capable of playing through the wing as well as the middle.
So the:
Neymar----Messi-----Dynamic CF formation can quickly change into:
Neymar-----Dynamic CF-----Messi or:
----Neymar---Dynamic CF----
---------Messi------- or even still:
----Messi----Dynamic CF-----
--------Neymar--------