I read somewhere that Doc said he was going to use the first 15 games to establish his rotation. Walker should start to see some PT in a week or two.
Also keep in mind, Doc has a tendency to sit his young guys early in the season. It happened to Powe, Gomes, Rondo, DWest etc. Bill Walker is probably getting the same treatment. Right now he is paying his dues and putting in work on the practice court. He'll get time soon.