If we're being absolutely ruthless I think 2 players are critical, Pau and Watkins. I don't think we can pick up a CB or CF to replace them. At the same time we would I also know we would adapt.
This season is the example of how good Unai is, he has reinvented the side 3 times with the same principles. Lost Mings = brought in Pau, Bailey starts playing well = adjusts the side for him instead of Diaby and finally lose Kamara/2 of his other CBs = McGinn in with a flat back 4.
Each iteration of our team has required a change in how we're playing.