Messi plays RW but drifts in the center often, that is why he is helped by the presence of a strong striker who could draw markings on him and allow Messi to attack the opponent's backline frontally, comming from the 2nd line. Suarez, Eto'o, Ibrahimovic those guys needed to be double marked, which meant more room for Messi.
What I'm suggesting is simple. Messi can't play false 9 now, because he is older, does not have Xavi and Iniesta duo to combine with in short spaces in midfield (which is the reason why he played false 9 in the first place, to combine with Xavi and Iniesta in attacking midfield areas), and the team doesn't have a plan, or the players (or both) to fill in the space left by Messi in the right, and pose a serious threat on that side once Messi is gone from there.
Suarez is a clear no9, not a winger. So Messi can only play RW in a 4-3-3 system. And playing with false 9 is not a tactical setup that you only use when Suarez is not available, since it changes a lot of things in how the team is supposed to play from midfield up.
The question is how do we play Messi in RW, and more specifically, who do we have there to attack the space Messi leaves when he goes more centrally? Nobody who could trouble a good team, which is why Marcelo had absolutely no problems in defence in the clasico even though he is mediocre at best at defending. Let's be honest here, nobody is worried by Sergi Roberto. He is not even half the attacking threat that Alves was. Which is why we probably want to sign Cancelo.
