This is a cop out, but I think a lot is context dependent.
By that I mean, what role does the team need their SG to play. A microwave scorer? A solid wing defender? A classic 3-D guy? A combo guard who can take some of the ballhandling load off of their bench PG?
I think the Celtics need someone who does not turn the ball over, can handle the ball adequately, hit open threes, and play solid team defense. Baylor seems like he can do those things effectively.
I don't know what Joe and Co. think they need out of their last starter. It will be interesting who of Hauser, Scheierman, Hugo, and Walsh gets the nod. Last year they rotated that starting role around.
But to answer your initial question: I think he is middle of the pack bench guy who still can improve to something better.