The mainstay players from Barcelona are not able to play consistently because it requires very high energy to guard at the back which is very vulnerable to opponent attacks, Many defenders are injured because of the previous game, therefore Xavi must carefully consider the formation for each game so that get good results.
Actually I see two weaknesses of barca at the moment. They have Pique already starting to slow down and midfielder Busquet who is starting to age and xavi has yet to find a replacement. Xavi understands very well his young players are not experienced enough to be alone without them.
Maybe Xavi will take advantage of the winter later to buy new players. They need an experienced young midfielder and central defender.