Luiz, 25, slotted in to partner Mikel for the 3-1 victory on Thursday against Monterrey in the Club World Cup.
He has played there before as a kid but Mikel was surprised to see his pal next to him at kick-off.
Mikel said: "It was a bit of a shock to see him lining up there. I said to him on the pitch 'where shall I play?' and he said 'I will play the holding role and you go forwards'.
"I said 'yes, OK' but when the game started I looked up and he was up their end of the pitch.
"I don't know if it's going to be long-term though. I think he needs to play at the back."