This might be true, Grealish is a defensive winger, Like Dirk Kuyt,Valencia Park Ji Sung, his main role isn't G/As. I agree that in big games Grealish might get the nod because Peps main thing is control. However if we have the added protection of Stones playing alongside Rodri is prefer to see Doku start ahead of Jack.
I dont think Doku is the reason we concede more. He doesn't lose the ball in bad areas, most of the time its either a cross/pass/shot in the final third and to be honest thats what a winger should Do, trying to create/take risk/make things happen.
againts low-blocks he should always start, in bigger games if he can improve his end-product I think he starts too, until then it will be rotation with Jack.