Rodney Miller HAS to play. We have no other options at C. This really isn't that complicated.
(Yes, we know, we should have other guys on the roster yada yada. But we're in-season now, and we don't have other guys. You can resume beating your dead "roster management" horse in a few months. No one disagrees that we should fill the scholarships, but your shtick is tired and boring, especially at this point in the year. Just accept where we are and move on.)
Back to the matter at hand...by rule, we need 5 guys on the court. There is literally no one else on the roster to give Rodney's minutes to. It's impossible that he will be below 15-20 mpg. He's a bit above that now, but not by much. His minutes will likely decrease a bit over time, but he will play significant minutes all year long.
Regarding Walker, when he has been on the court this year, our offensive rating is a pathetic 91. He is a much bigger liability on that end than Rodney, whose offensive rating is 116 (good for 301st in the country, for whatever reason). Walker's offensive rebound and defensive rebound rates are both 6.8%, which are both significantly below Rodney's (as well as Sam's). His assist rate is lower than Rodney's and his turnover rate is higher than Rodney's. Walker's block and steal rates are impressive, and are highly encouraging athletic indicators for the future, but that doesn't mean he's ready to make an impact in the paint today.
None of this is to say that Rodney is a good player, or anything close to that. But today, Walker is even worse than Rodney, by any measure. Your argument that Rodney is "insanely inefficient", when compared to Walker, is completely unfounded.