They need to be able to do the bare minimum: make approximately correct defensive rotations, don't get beat in transition, run the plays correctly, hustle without getting in the way and bring one skill to the table, be it defending, 3-pt shooting, drawing fouls or a good mixture of the 3.
Otherwise they're not going to see the floor enough to obtain the experience that Doc needs to trust them down the stretch, and thus we wouldn't be able to expect much of them.
Bill Walker needs to make a big leap. I think the rotations and execution will improve incrementally with the passage of time and the increase in familiarity but he still doesn't have a concrete skill. He doesn't penetrate, shoot or defend any better than Tony Allen at this point. I think he should work all summer on the corner 3, the pull-up 17 footer after a close-out and finishing with his off-hand. If he can get up to NBA snuff in those categories, I think his marginal improvements in other areas will get him some floortime and possibly enough run to make him useful for the playoffs.