Dunno, think you should watch Sevilla again, especially last season under Sampaoli.
Physical stature is irrelevant for playstyle btw.
I've watched Sevilla plenty in the last few weeks mate, covered both of their semi-final legs v Atleti and their league games with Alaves and Espanyol.
N'Zonzi was put back in after the Alaves defeat, in which Montella started with Pizarro and Banega in midfield (as the manager before him was doing).
N'Zonzi's presence in the middle has given them a big lift - his size does make a big difference to how they play, because he's the pivot that the rest play around.
Montella is finding a system that works so I'd be surprised if Mesa was a direct replacement for N'Zonzi. Seems more likely to be an alternative to Banega imo, unless Pizarro is moved on - though I'm sure he only signed for them last summer.
They've had to change from what they were doing under Sampaoli because Montella is there now.
Sandro will probably do well for them, but hopefully that move has given us an in with them for N'Zonzi.