All well and good having a good number 10 behind the striker but there's still the issue that Gibson aside no one playing in the deep two midfield positions can pick a forward pass. Either Martinez is waiting for Gibson or Barkley needs to be moved back. One defensive mid, one deep playmaker, and a number 10. If you want to play 4231 that is how the midfield needs to be structured: busquets xavi iniesta, Khedira Schweinsteiger Ozil, De Rossi Pirlo Totti, even Mascherano Alonso Gerrard to some extent.
Instead we currently play two holding mids, neither of which are a true DM with McCarthy being box to box and Barry more of a schemer. We then play a box to box midfielder in the number 10 position. Martinez needs to pick a DM put one of Barkley or Gibson next to him, then buy a number 10 if he wants to play 4231.
Martinez has said that the #10 he wants to buy will drop as deep as Barry and McCarthy - hard to fathom exactly how that would line up but perhaps that would address the problem?