He's played on the right side of MF most the of season for Celta and has done really well. Like Flavia mentioned he's very versatile. The only reason why most people think of him as a forward is because Pep/Tito played him there because of his dribbling skills. I hope Tata uses him in midfield when he returns.
mertesacker and Hummles are both too slow,
You aren't going to find anyone with everything except for one or two players who are impossible to sign. Hummels would be fantastic. Very clean defender and intelligent.
David Luiz is very good option.
Fast, great in the air, expansive range of passing, similar hair to Puyol.
Only downside is his occasionally erratic behaviour, but that won't seem too apparent in our defence
The hair option is the most important one.
But to be fair Luiz isn't a top defender.
I like him cause he can play pretty well in DM and has a great long range shot, would probably be good for rotation when we have Bartra/Masch and Pique in cb and we want to rest Busi or Xavi.