How I visualize the roles in this midfield.
LCM (Iniesta Role)
Iniesta/Arda/Rafinha
DM (Busi Role)
Busquets/Roberto
RCM (Rakitic Role)
Rakitic/Roberto
Well I think everyone here shares the same thoughts anyway. Roberto has also played with with a Roberto-Busi-Rakitic Trio as well, But These roles is what I assume if I/R/B need to come off during a match. When Rakitic comes off the first one that comes to mind is easily Roberto as he's also a Box To Box mid that can slid in Raki's role when needed. This is where Lucho made the mistake, Arda hasn't show he could play that role as you can see during the Classio and other matches during this year.
Lucho, without a doubt deserves a lot of Credit for reviving Roberto from the depths, but ever since Arda became Available I feel like he "Forgot" about Roberto's main strengths as a Midfielder. Arda Just cannot play Rakitic's role Like Roberto could and it surprises me Lucho persists with Arda coming off the Bench first for Rakitic and Not Roberto.
Arda is better suited coming off the Bench for Iniesta, and Same with Rafinha. This isn't a Dick measuring contest on who's a better midfielder between Roberto and Arda it's more who suits the role better if we need Iniesta or Rakitic to come off.