The nominated "captain" should really be a figurehead. There should be 4 or 5 leaders on the pitch. Take the starting XI in Istanbul; the likes of Eddy, Ruben, Rodri, Kev and Boulders (along with Ilkay) would all be expected to lead (by example) on the pitch. I'd also include Kyle, of course.
As a figurehead, though, it'd have to be Ruben. He just comes across as a leader of men.