He's a legit left back. Would have rather had Danilo, who has more leadership (and earns less disciplinary cards) and can create more going forward (including scoring goals). But he's more a RB and we wouldn't pay Real Madrid lunacy pricing anyway.
Sandro could be a starting LB for many years to come. So a good play.
(And no, Marcelo can't defend for $#@!e... don't see him as the same player.)
And we saw how badly Porto went to crap when they played Bayern in the CL with Danilo + Sandro vs. without them.