Obviously you skew the numbers significantly if you include a 2 minute game, but that said, Brown is banged up a bit. The team though goes as Tatum goes and always has. Tatum is the guy the team needs to be playing. Brown is almost irrelevant to Boston's record.
The Celtics have a .467 winning percentage in 15 games without Jaylen this year and a .635 winning percentage with him.
Your numbers are wrong, the C's are 8-7 without Jaylen playing at all and also won the 3 minute game, so are really 9-7 (56.25%) without him. They are 31-20 (62.0%) with him playing a full game, which is obviously better, but not nearly as much as you might imagine. And it was the Western Road trip in early December that really skew the numbers as Boston went 1-4 on that trip all without Brown (that was the 5 games in 8 nights with 2 separate back-to-backs).
I counted and have now recounted 7-8. Going through the schedule maybe I mistallied, but I don't think I did, and I recommend you count again. And I'm not sure how the West Coast trip skewed anything -- having Brown for those games might mean the C's would have won 1-2 more of them, particularly one of the games in LA, which is exactly my point that he does matter.
So I'll use your accounting method with regards to giving him no credit vs. Atlanta. That's .500 compared to .627, which is the difference between 8th place and 2nd place in the East. That's a really big difference, and with Jaylen having missed 16 games, it's not merely small sample size.
Tatum is more important, I'm not arguing that. But that doesn't mean Brown is unimportant.