Pep could keep both in reality don't you think? Roberts already having CL experience and didn't have that bad of a pre-season as some made it out to believe should be fine with him. Theres going to be plenty of PT if we don't bring in Sanchez/Mbappe or have a plan C/D.
We seem to be well stocked in the positions that Roberts plays. I'm not sure he would get much game time, apart from domestic cups, unless we have major injury problems. Because Foden & Diaz can play a bit deeper and central they complement the current squad a bit better. Roberts is perhaps ahead of them in terms of his development but he's just unlucky that we've less of a need for him.