I think the choice between Doku and Grealish on the left is very simple. Pep has the advantage of having two completely different profiles of players. He will use them depending on how opponents set up against us.
If we play against a low block, Doku is the better alternative on the left. His ability to create from the left when teams focus on the middle will only make it impossible for opposite team's block to stay compact. Grealish is good enough to play against low blocks and offers more control, but Doku's unpredictability makes him more valuable in those situations.
If we play against a team that presses us high, Grealish is the best alternative. Pep wants Grealish to play a similar role to Iniesta, which is helping the build-up from the back. He's the best at it and nobody (besides Bernardo) has the spacial awareness and ball retention to do it as flawlessly as Jack. I never understood how people watch us play and still judge Grealish on stats. His skillset is rare enough for it to be obvious to the untrained eye.