If Xavi gives every new player a chance to show himself, Barcelona will never regain its former power. The problem is that to understand whether the player has adapted, whether he shows his level of play or not, you need at least a few matches, that's a lot. I think he will assemble a team of players in whom he is confident and who are time tested
When it comes to giving new players the chances to show case themselves, it often comes out beautifully because, you would see them putting in there all in the team to make every effort count. There might be some flaws due to inexperience because that does work too but fir sure, there all would be show cased on the pitch. To enable them win a first team shirt or at least be on the bench. You can take a look at Arsenal and its young team, they might not be the best in there prospective league and are weathered by inexperience but, they are doing ther quarter and it's helped them to the Europa qualification.
It's probably best to give them a chance to prove themselves in games that don't matter, so I mean it's not a good idea to let newcomers test them in key games or against average opponents, especially if they haven't played before. So the right decision would be to let them on as a substitute in the second half, this way I think they'll adapt and show what they can do in a few games