Again, horses for courses. Delph is the real deal. Yes, he plays the role differently, but he is equally effective. Some will suggest more so.
Delph is in fact the other starting fullback. Not even a backup. Against WBA Mendy and 2 strikers might be the way to go. Against Spurs, Delph and 1 striker might be more appropriate.
The notion that Delph is an inferior non-leftback is a silly fallacy. He is a legit left back with strengths and weaknesses just like Mendy. For fuck sake, he is currently the best leftback in the league. Mendy included!