Considering his minutes per game, every number here is solid with the exception of his 2pt fg % which would obviously improve in a system where he'd not have to stand around watching Roy pound the ball into the floor and actually get some open looks.
No matter how you slice it, a career .400 from the field is a horrible shooting percentage. And that includes a jaw-dropping .370 during the last season, so the guy has regressed in this parameter.
That this will "obviously improve" is your wild guess. Rudy Fernandez is, at best, a complementary bench player on any team, so if he doesn't know how to be effective in this role in Portland, there is no reason to believe he will be any better in Boston.
The guys makes $1mil a year. Whose stats should his look like -- Dirk's? Are you seriously comparing him to Michael Finley? He's 25 -- his best years are ahead of him -- not 6 years behind him.
What fans (and even some GMs in the NBA) fail to understand is that being young is not much in itself. What differentiates good teams from bad teams is that good teams are able to acquire what they need when they need it. At this point, the Celtics need a bench player who can hit open shots for 2 years, and as much flexibility thereafter. It doesn't really matter whether the said player is 25 or 33 today, as long as he can play the role he's needed in.
So the fact that Rudy Fernandez is not old is really not relevant. Or the fact that he "may be good in the future" -- he's not going to be a player to build around. If he were, we won't even be having a discussion of trading him for a protected first-rounder. Someone like him can be found as need arises, and when more critical signings have been made.