Teams he has been in have been successful - both Barcelona and Valencia have been champions whilst he has been a regular player. You dont have to be lightning fast to be a good defender (helps though imho). If you have got a good positional sense then you can get into these positions to stop the attackers.
The Argies arent known to be soft defenders tbh so could well be a good signing and help Hyppia and Carra have a rest now and then + Carra can play def mid so this also maybe an option Rafa is looking at.