Casillas dropped a clanger in his return to the Champions League. The Porto goalkeeper could have done much more to prevent Besiktas' second goal away to Porto.
The ball slipped through his hands from a fierce shot by Cenk Tosun, putting the Turkish outfit ahead in Portugal.
What a strike from Tosun #besiktas #PorBes #UCL pic.twitter.com/MZFzFeKpRt
— dave (@DesTaquito1) 13 September 2017