Foyth isn't as weak as some people think he is.
He's not van Dijk level strength, of course not. But he bullied Michael Antonio in the League Cup and he's given plenty of more experienced defenders a hard time. I've seen Sanchez bullied more (though of course he's played more)
If he isn't going to be playing week in week out, then yes, he needs a loan. If Mourinho is happy with Alderweireld, Sanchez, Dier and Tanganga as our four centre backs, then loan Foyth out and give him a full season of games and then see where we are at the following year when Alderweireld is another year older etc.
I think with all the games we are going to get coming thick and fast at the start of the season it'll be worth keeping him at least until it settles down slowly. We have until late October I believe to loan him out.