He's just not there as a playmaker. He's a scorer by nature and I'm not sure he'll ever have the playmaking instincts to be the primary playmaker on a title team. Now, you get a good playmaker alongside him, ask him to be more of a scorer, stay off-ball more, and be a secondary playmaker then I think he'll really shine.
He had an atrocious 24% turnover rate and only .78 PPP as a pick-and-roll ballhandler this postseason. Not just the Finals, the entire postseason. Both of those numbers are very bad. That's not even getting into the eye test and just how bad many of his turnovers were.
It's great that Boston is expanding his game and pushing his limits but there comes a point where you're square-peg, round-holing it. Not that this flawed Boston roster had any other playmakers to turn to.