To be fair. Villa aren't England, and Villa aren't playing Italy.
There's an argument to use Grealish early on, and hit them hard, quickly, with pace... and if we get a decent lead, to shore things up and possibly take him off then.
Conversely, we can save him until their legs are tired, and hit them hard then... when he might have even greater impact (and we might also need him in extra time).
This is true quite a few players (Foden, Sterling, Mount, Walker) - players with pace that are highly likely to trouble tired legs.
I don't envy Southgate. It's not an easy decision to make, but I personally think we have a fairly decent formula, and it makes more sense to save Grealish and Foden for later.