My take about average level of age, if this is the main team (without counting goalkeeper):
Alba, Masch, Pique, Alves, Iniesta, Busi, Xavi, Messi, Neymar & Alexis (4-3-1-2 anyone?)
The average would be 27.2 yrs, with neymar being the youngest (22), followed by busi (25). the oldest of course xavi (34) followed by alves (30).
This composition might not be the youngest, but it's not that old also. Does xavi needs replacement? in the future yes of course, because he's already 34. But at now, he is still our conductor, no one really ready to replace his position. Cesc? I know he has great vision and great pass, but to see the team as a whole, to know when to shut it down or to get patient, awareness of teammates condition during the play, i still think Xavi does it better. He's like the coach's eyes on the field.
But in the end, you can't fight with age. I think he's still capable for around 2-3 more years. Until then barca has to prepare the appropriate replacement.
Player like Neymar, Alba, Busi, Pique and of course Messi, still has many years to play. They're still our future for at least the next 3-4 years. Fabregas and Alexis are also strong players, if you could use them to their full potential. Fab has the potential to be the next Xavi, i think, other than making Thiago return, of course.
Also, Masch need to be place at his original position, as a quality replacement for Busi. The central defender is for someone bigger, taller and at least as quick as Pique. Experienced defender is a nice option. Nothing wrong with defender around early 30, as long as he is capable and has good experience facing variety of attacking players. We still need Alves since he still got the speed and cross accuracy. Bringing in a replacement should work, only if the guy is proved to be better than Alves, and not only younger, and has a better sense and discipline in defending.
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ps: I know this is Messi's thread. Just to make people realize that the fault is entire team, including coach. Not only Messi. He may have bigger chunk due to the camera focus during his pedestrian minutes, but we know he was already like that from at least 2 years ago, and look how 2011-2012 & 2012-2013 went. Even this season, except some games surrounding his injuries and several matches, which some of them turned out be critical matches, like the one against atleti.