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 shite... 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.