All good points on this thread and I don't necessarily disagree with any of them. However, the issues with the Jays would not be so noticeable if there was a legit third star alongside them. This can be done actually rather easily, but trading Smart and/or Timelord is required. Done, problem solved.
However, I personally don't want to do that. If I'm Brad Stevens, my goal is a starting lineup of:
Smart, Brown, X, Tatum, Timelord
Who is X? As others have pointed out, it's not really a specific position to fill, like PG or PF since Smart, Brown, Tatum can all play several positions. Just get that guy and figure it out from there.