Not sure on Mainoo himself but you're alluding to Armstrong being brought into the squad next season.
For me, the 2 central roles are LCM: KDH & RCM: Garner.
At Derby he would be deeper so I suppose could push Garner for a place, while at Preston the 2 games ive been ready to watch recently - hes been injured - though im told he's more advanced so more like KDH.
So if as expected he returns to the squad in the summer we'd need another CM in there from:
Rohl
Iroegbunam
Gana
Would you have those over Rigg or Mainoo as part of the 4?
I still don't think Garner should be our starting 6 next season.
If
KDH moves to 8 full-time, then we want an absolute beast of a DM next to him, which Garner is not.
Armstrong should be getting ~1500+ minutes at CM next season, whether that's at 6 or 8, which doesn't really leave room for another 19/20 year old CM.
Basically - buy a top class number 6, then we can have any of
KDH, Armstrong, even
Rohl if he comes good next to him. Garner can cover both positions and right back ably.
Keep Garner at 6 and then you have a 6 who is a good all-rounder but not elite at any of passing, ball-carrying or ball-winning. So then how do you balance that out in a pairing? You're always missing something.