The idea does not lack sense. Matic is more of a deep lying playmaker than a DM. That's something most people miss. People try to lock him into a DM role but he is not at his best playing that role. Meanwhile, if Fabinho is not playing right back, his best role is as a DM.
Moreover, compare Madrid's midfield with United's midfield:
Real Madrid's Midfield
Modric
Asensio
Casimero
Isco
James Rodriguez
Kovacic
Kroos
Manchester United's Midfield
Pogba
Carrick
Mata
Herrera
Fellaini
Mikhi
Lingard
Timothy Fosu Mensah
Replace Fellaini and Fosu Mensah or Carrick with Matic and Fabinho and suddenly we are looking better. Replace Lingard and Mata with James Rodriguez / Dembele or Pereira and suddenly we are looking great.