One thing I will say is that we looked somewhat good the first 20 minutes (pressed well and had even a few chances to score) but as in all the other games the team was dead tired after that.
Problem is that physically that team cannot go much longer with this. Your team has a physical problem first - not a technical or even tactical one. 20 minutes is not long enough. Taking the full match time your team made 162 pressures (won 24 %) whereas the Bayern team that played for nothing had 214 pressures (won 28%). It is about endurance first or workrate. I am not sure how Xavi can change that in the season and if you might lack physicality in the choice of available players, too.
But I do not think that there is easy solutions. If you look at Bayern - they have a very good mixture between old an young - and the older ones are physically in great shape and are still the leaders on and off the pitch. Lewy is the superscorer - and M?ller the one that coaches his offensives and leads the press. You need the older players because of their experience and their influence in the dressing room and even in crisis situations where you send them out to deal with the press - but you need them in good shape. They do not have to be the fastest - that you can substitute with strengths of the younger ones - but they have to be in a condition that they are in top form individually so that they never get really questioned.
I am not so sure that the older players you have are really the ones made to be team leaders. I even think that having to much focus on Messi and not balancing that out with others (like it was before with Xavi and Puyol) was a problem in the recent years, too. In my eyes it is not the artists or individualists that should be the captains or team leaders - yes, they should have a big role, but others need to be as important and the problem even during the last seasons was that there was nobody who stepped into the roles of the players that left.
That is just an outside view from me. As you know I am an Bayern fan - but one that often watched Barcelona matches before.