He is fine - as a defensive left back.Mitchell's fine. What he needs is a genuine left sided player in front of him - Munoz has Sarr on the right and it works, Mitchell has Eze, not a natural left sided player. I agree with Lanzo here, Eze should be plying his trade centrally, please note his positioning for his goal. Because of Glasner's tactical approach, some players don't seem a natural fit.
However, he's not fine as a wing back in Glasner's system where the wing back is there as the spare man out wide to provide the attacking outlet.
He has a couple of assists this season, but the role demands more than this.
He rarely hits a first time cross and almost invariably stops the ball , turns and passes backwards with the ball eventually ending up either with Henderson of one of the centre backs.
I observe Glasner on the touchline urging Mitchell to drive forward, so it's not that Mitchell is observing Glasner's instructions.
I agree the situation would be eased if there was a genuine left sided attacker ahead of Mitchell, but
a) we don't play this way
b) we don't have one
c) the left sided role is the only one available for Eze in the current system, so he is kind of shoehorned in there.
This is not a criticism of Mitchell.
He's fine against the top teams where defending is paramount.
However, against the weaker teams it would be better to have a more attacking outlet in that left wing back role.
This could be provided by a new player who can compete with and share the role with Mitchell so we have another option.