Yesterday was the first time he has come on as a sub and been lacklustre (in recent games he's featured in), and you have to expect some up and downs in form at that age.
Overall he's impressed me more this season than the times I've seen him before. That said, being a PL starter is a different ask and I think he'll be used mostly as a sub for a while yet. Although he'll probably get the chance for another start over the Christmas period if the games are coming fast.
I think Silva has used him well so far, particularly given he had almost no pre-season.