First of all, I think ESPN puts too much stock in their RPM stat. I'm not so confident in its efficacy, at least not yet.
Second, sure, rookies are bad. They typically hurt their team's chances of winning.
When people get excited looking at players on, for example, the Sixers, it's not really about how good they are now, but rather the potential they have to make a big impact in the future.
This is a dimension that's lacking on the Celts' roster.
Also, you could make a strong case for Marcus Smart to be on the list of "slumping sophomores."