I think Sanders struggles too much to get into the zone right now for him to be a starting option. I'm with Bertz; I wouldn't mind getting him a look in the BP: give him some bulk low-leverage innings at the start and see how MLB hitters react. He's doing a really good job of getting Triple-A hitters to chase and whiff, but sits in-zone in the 10th percentile and when he's getting hit, he's getting hit hard.
Triple-A hitters are just not anywhere near MLB hitters. Think about how good Ballesteros has looked time and time again in Iowa and how much he's struggled in the last month. I've read more than once on here that he's still the "best pure hitter" on the team or something along those lines from people here (for the record, I think maybe he can be that, but he isn't today).
Sanders has probably done enough, coupled with the terrible Cubs' pitching injuries to force a look that way. But I think as a SP, MLB hitters are going to either use that low zone-% to their walk-advantage or force him into the zone too often and hit him hard. Shorter spurts make it easier to hide poor zone-rate. And low-leverage situations will allow the Cubs to see whether or not he can continue to get whiff and chase against far better and more seasoned hitters to offset the zone issues.
I know we're frustrated right now, but the Cubs are above .500, in the playoff hunt and the Brewers are not an impossible number of games away (the Brewers were roughly this far back from the Cubs last year around the same point of the season, so it can happen). And to be fair to the Cubs only the Dodgers have gone 7-3 in the last 10. They aren't in "just start the kids" territory yet. But with injuries probably are in "well, might as well see what Will Sanders can do down 4 in the 7th inning" territory, too, rather than fill that spot with our 5th recently DFA"d 30 year old journeyman we pick up.