In my opinion the first priority is Cesc, without any doubt (without selling Thiago).
Cesc is the best midfielder in the world after Xavi and Iniesta, he's only 24 years but
with a lot of experience in CL and in EPL. Sahin, Pastore and others are not
even comparable to Cesc !
Cesc is expansive, but we were ready to spent 40 million for Villa, a player nearly 30 years, between 2 or 3 years at latest we have to think about replacing him. Cesc in 3 years will have only 27 years!. Cesc is a very important investment for the future and a very versatile player, that allows us to play in different ways.
Cesc is the key player for the midfield, rotation with Xavi and Iniesta, but not only.
Does not have to be always a substitute for Xavi, we should not always play 4-3-3, we have players with so much talent that allows us to vary, for example a 4-1-3-2:
Alves-----Puyol-Piqué-----Abidal
--------------Busquets
----Fabregas-Xavi-Iniesta
----------Messi------Villa------
Thiago has only 20 years, and if Iniesta and Xavi are injured for 2 or 3 months, Thiago is not yet ready to replace them, Cesc yes.
Injuries of Cesc? for me they are just unfortunate cases, not chronic.
Thiago will still have a way to play more, but he has only 20 years. We can not
expect that tomorrow he will become a genius. But Thiago has the opportunity to train, learn and play with players of a level of Xavi, Iniesta and Cesc, an opportunity that happens once every 100 years! And in a couple of years he will probably be el fenomeno.