We get this a fair bit on this forum. If Pellistri isn't playing, it's probably because ETH and his coaches watch him every single day in training and have decided he's down the list in terms of options - not through some kind of favouritism or unwillingness to try young players.
Remember training isn't just drills, there'll be all sorts of mini-games, competitive 11 vs 11s, 5-a-sides etc...its perfectly possible to assess a player based on all of this body of evidence, as an alternative to having to throw them in a Premier League game.