If the Celtics add another wing during the season, and Brown continues to handle the ball as much as he has been so far this season, I think the Celtics will investigate the trade market for Smart.
The Celtics seem determined to play Tatum at the 3 and Brown at the 2 if possible. I think that's what this 2-bigs starting line-up is all about. Brown may start eating up a lot of the minutes at the 2 position, cutting into Smart's minutes once Walker is back.
Smart will be on an expiring contract next season. The Celtics may want to avoid negotiating with him again. It got a little messy last time. He's an excellent trade chip right now.