I don't think you can just dismiss what he's done this year. He looks slow and his YPC show that. It would be different if Lacy's YPC wasn't very good, but when Lacy is at 5.5YPC and Starks is at 1.8YPC, that's a pretty sizable difference.
Just looking at the last game, Starks was 12/33. Lacy was 11/81. Lacy had 48 more yards on 1 less carry.
I don't think Starks is all that great. Even last season, his YPC was the same as Lacy's. I'm not running out to pick up Starks and I have Lacy on 2 teams.
No one (at least me) is dismissing Starks' performance this year -- had even mentioned it in my post.
But I think your focus on a single game, or just YTD, is a little too myopic and fallible in that it wrongly discounts what Starks has specifically done in relief for Lacy when he was out. If your focus is purely on YPC, a just as compelling argument is that Starks is not much different than Lacy, with only 0.2 YPC difference over the course of their career. That approach has just as much fallacy to it as looking at a single slice of data from the whole -- Starks is no Lacy, never has and never will be.
If I were to reframe this in terms of probabilities, there are a few options going forward:
a) Starks can't hold Lacy's jock at all -- he's looked slow and worthless and can't pick up the slack
b) Starks will show what he showed last year when Lacy was out - he's capable of carrying the full load and be successful doing it
c) somewhere in between
If we were allocating 100% across these options, I put more more probability in b than a -- Starks has proven himself before, and I can forgive him looking slow against the Giants who, aside from their game against the VIkes, have been really stout against the run -- 9th in rushing yards allowed per game, seventh in least fantasy points allowed to opposing RBs, only 3 rushing scores td.
You choose to put more probability in a. I think history (outside this last game) has shown differently.
I think not having Starks as a cuff to Lacy is unwise just because he's the only guy there who can carry the load if Lacy is out for any extended period of time (Ripowski is surely not that guy). If you have other options, great, but I don't think it's a stretch to say that Starks -- even if he is pedestrian compared to Lacy -- is a great snag given his ability to carry the load (and his ability to do it well in the past) if Lacy is out.