Mascherano is decent at CB, but he's still an emergency back up. I think that next season we should start a back line of Alves - Piqué - Abidal - Adriano. Honestly, as great as Puyol has been in the two clásicos, Abidal was fucking amazing when he was playing there during the captain's injury. If he comes back healthy he needs to start. I prefer Adriano at LB anyway because he has the potential to give us the same offensive thrust as Alves and pretty good defense as well. He's faster and stronger than Maxy and he seems to have started to get the tiki taka style as his playing time increased. His only problem is that he can't run all day like Alves, so we need a good back up for when he gets tired or injured. Honestly, I prefer Puyol to be kept in reserve to fill in for the inevitable injuries and suspensions as he can play all 4 spots. If we keep Maxy one more season (or if Bartra is ready for the first team) and Fontàs can give us minutes in the Copa del Rey I don't think we need to sign any more defenders next season.
Check out the depth chart:
RB Alves, Adriano, Maxwell, Puyol
RCB Piqué, Puyol, Busquets, Mascherano
LCB Abidal, Puyol, Fontàs, Busquets, Mascherano
LB Adriano, Abidal, Maxwell, Puyol, Fontàs
That's 8 players who can all play at least two positions to cover 4 spots.