I'm wondering if Robert sees him as a potential longterm replacement for Pique. It seems he's a similar type of CB to Pique and they probably won't be the best fit to play together. Maybe he'll surprise and develop quickly but at 18 he's probably 2-3 seasons away from having a place in our first team. While I think Pique with his playing style can play at top level until 34-35 if he stays healthy it's probably time to start looking and trying out his potential replacements in the future.
So that probably means even if Marlon is promoted (and he could get loaned out) we could still go for someone like Yerry Mina, Davinson Sanchez or Lemos in summer, because we'll most likely need 2 new CBs over the next season and half with Mathieu and also Masche possibly leaving. Although I'm not sure if they (Mina, Sanchez, Lemos) really are as much more developed and experienced at higher level than Marlon that would negate their adaptation period in comparision to Marlon who has already trained and also played some games for Barça.