Celtic fans like to hate on their players.
I actually think Boston Fans tend to overrate our players.   The rest of league knew that Sullinger gassed and a lost cause before many here did.   People always think that our free agents are going to command a lot more than they do.    Haven't you ever seen some of the lop-sided trades that are proposed here?   Or the talk about Yabusele being the next Draymond Green.   So I clearly think that the opposite is true.
This is nonsense.  All fans of teams rate their players higher than players on other teams.  It's called bias.  I am sure Magic fans think Elfrid Peyton's hair looks good.
As for Rozier, he is a 23-year old on a rookie deal who at this time is a plus defender and elite rebounder for a PG.  His shooting is not good enough and he is not a creator.  The question is:  will those deficiencies improve?  If you are a fan of his, you are betting yes.  If you are not, you are betting no.  That simple.
I think most Celtics fans are of course fans of Rozier and therefore think that he still has room to improve.  That is not overrating.