The current Plymouth manager admitted that he would pick Mainoo over Trent Alexander-Arnold in England's midfield.
The Liverpool right-back is hoping to gain a berth in the starting line-ups at Euro 2024 but in midfield instead of defence.
Rooney said on The Overlap: "No. I have watched Trent play in midfield a few times and I think you lose a lot of his qualities. His delivery is obviously incredible and of course he can pick passes from midfield, but I think he has to fight it out (with Walker for the right-back spot).
"I know Kyle is the best right-back in the world at the minute but I think midfield is such an important position. I would play Mainoo before Trent in midfield, 100%.
"It's not that Trent can't play there, we saw him do it recently against Everton and some of the passes he was hitting were unbelievable.
"It's probably only Becks (David Beckham) who beats him in that area. But I think to play in midfield, you need that discipline, especially when you come up against the bigger teams, and he might get found out a little bit."
