I disagree. I think, in a pinch, Denver always knows it can trust its running game in ANYBODY's hands. Look at Portis and his huge fumble-bug in his first couple games last year- that didn't stop them from trusting him. Plummer, on the other hand, is still very unproven, with one great game, one horrible game, and one game where he got injured before halftime.
I think it's more along the lines of Shannahan screwing with the heads of not only the Lions, but every team he faces for the next couple of weeks with his wacky playcalling. I look at it very much the same as I look at Spurrier calling something like 2 pass plays in the 3rd quarter, 4 total in the second half.