After Tracy took over the lead role last year in week 5, he was RB14 over the rest of the season. And that was on a much worse Giants' offense than they've had since Dart took over.
The first half of those games after he took over the lead role were even more impressive, as he was a mid/low RB1 averaging nearly 5ypc and catching lots of passes. His first 6 games as a starter actually outperformed Cam's first 6 games as a starter in every metric except TDs and energy. And again, on a much worse Giants' offense.
He hit a bit of a rookie wall at the end of the year, but this year gets the advantage of starting his lead back role in week 9, so that's probably less of a concern.
Again, Cam is fun, but I'm not really convinced he's some incredible RB that can't be replaced (outside of energy). If you take out the TDs, which were just much less plentiful for the entire team last year, there's really been no difference between Tracy and Cam's production as the starter.