Does it really matter if the Celtics sign Mack? I doubt he will play much. I assume Lee is going to pick up most of Barbosa's minutes since he's only averaged about 20 minutes in the past three games.
Mack's played plenty of games in the NBA. He isn't very good. He's a shooting guard in a point guard's body but without the skill set or athleticism to play either position well. Despite what he says about defense, he's fairly slow for a guard and not a good defender. I can't see Doc looking down the bench and going, "Yeah, I need Mack to handle the ball or defend a position."
While I agree with your analysis of Mack, the sad truth is that having doc look down the bench and see Mack there is better than having doc look down the bench and see no one there. Depth is important during a long season in the nba, and we can see why now.
No one is saying Mack will add significant contributions to the celtics, though we all hope to be pleasantly surprised.
Instead, at this stage a body is needed in case of blow out games. Why have lee or terry or Bradley playing during garbage time? That is what happened a few games ago.
And who knows? Maybe Mack will actually do well.