[neatoman]

While it is possible this movie will feature Turtles that started out as humans, if we're going to take the leaked concept art as representative of the final movie, then we already know Splinter and the turtles start out as regular animals. We can't really say "Oh look at how terrible the mutant whale concept art looks, I bet the Turtles start out as human in this!" because that would be selective of the evidence.

The concept art may or may not be representative of the final movie but it doesn't really work to focus the worst aspects of the concept art, and then also throw in conjecture that contradict it to make predictions about how terrible this movie is going to be. It's not exactly unlikely that it's going to suck, but at least go by all of what we have to make that prediction.