All the teams participating in these two competitions will be in the same situation where they have to divide their concentration, so this shouldn't be the reason why they failed in that competition. I think Xavi Hernandez already knows very well what he has to do to keep his players in good shape. He has experience as a player for sure he himself already understands things like this. Careful preparation and rotation will really help here.
Being a coach is totally different when he was playing as a player. So many critics have been coming for him early season caused by he has the worst history with barcelona. Xavi has worst win rate compared with another coach of barcelona in the same era. There shall not be a rotation when it will be only making the performance from the club to decline. The problem is MU was on fire. Manchester city as a strongest candidate in UCL has beaten by united. It was showing us about how strong MU right now.
Barcelona needs also become very careful. MU has a very good defenders right now and MU is has been found its soul. It can be said that MU has a very good team work now.