The numbers aren't great for the Celtics starters. As Kevin points out, they have "the worst offensive rating and the third-worst Net Rating of all 36 NBA lineups to play at least 200 minutes together." But really, it's an issue of player combinations and not necessarily who's in the game at the tip. For better or for worse (and the numbers suggest for worse), the starting lineup resembles close combat, trench warfare. Outside of Avery Bradley, there isn't a shooter of the starters that stretches the floor for his teammates. That's why Evan Turner is the most logical ball handler to lead that squad; with his size and ability, he can keep his dribble alive in the paint and he's tall and shifty enough to make a pass to a flashing Brandon Bass or cutting Tyler Zeller.
Some have suggested that if Thomas were to supplant a player in the opening five, it would be Marcus Smart. Sure, that would make sense because Smart is currently mired in a slump and Thomas is our best offensive players, but this is truly where combinations matter. Thomas' biggest strength is penetrating. After he's broken down a defense, he's surprisingly good at finishing at the rim, getting to the line, and finding shooters. Right now, that's not in Marcus' wheelhouse.
And look at the bench: Jae Crowder, Kelly Olynyk, Gigi Datome, and Jonas Jerebko. They're shooters that make Thomas' job easier and gives Thomas the chance to make plays for them.
Here's Isaiah Thomas' first possession from Monday's big win in Charlotte. Notice Zeller and Bass' movement. It's all pretty much key-centric and doesn't necessarily provide a lot of driving lanes for IT.
Kevin make a compelling argument that if Thomas starts, then so should Jae Crowder and I agree. They're a perfect fit. After Crowder comes in for Bass seconds after Thomas checks in, the two immediately go into their pick-and-pop game that leads to a Crowder drive and a Crowder pull-up.
The space is further accentuated when Olynyk replaces Zeller, Jerebko slides into the power forward role, and Crowder becomes a big 2. The floor is so spaced that it looks almost like a four corners offense. Thomas can play pick-and-pop or roll with any of the bigs. In the first half, he found JJ for an and-1 roll and KO for an open three ball.
Thomas finished the quarter alongside the Bass-Zeller big duo, but you can see how the spacing affects his game. In the first clip, he finds Bradley with the beautiful back cut, but that's more a matter of missed coverage than it is playmaking. In the second clip, you can see that Al Jefferson doesn't respect Zeller's outside shot as he retreats in the lane and forces Thomas to shoot an inefficient long-2.
With eight games left in the playoff push, a couple of losses might force Brad Stevens to revisit this issue, but this late in the season, you go with what's been working and ultimately, what makes sense. Small adjustments should be made in game and situationally, but these kind of broad strokes aren't fixes. If my steak is a little bland, I don't add a scoop of vanilla ice cream because vanilla ice cream is tasty, right?