This will be debated all season, and beyond.
You really think so?
Tatum missed 5 games, Boston went 2-3 despite getting Walker back for the last 4. They were 7-3 with Tatum to start the year and without Walker playing at all (and when they close out the Bulls will be 8-3 with Tatum and no Walker). And despite the Cleveland game yesterday, Boston is still 3.4 points per 100 possessions better when Brown is on the bench.
Tatum is the most valuable Celtic because he is the best Celtic and frankly I don't think it is all that close.
Genuinely cannot believe you're clinging so desperately to the laughable idea that we're a better team with Brown on the bench by using raw on/off. Funny stuff
And yet historically that stat has been a pretty darn accurate predictor of actual importance to winning. Look at champions. The Warriors, for example, Curry, not Durant, is what made them tick. Durant was a better player than Curry, but he wasn't more critical to the Warriors success. That was Curry. Without Curry the Warriors were a good team, with Curry the Warriors were an all time great team and that wasn't the case for Durant. We saw this quite well during the 17-18 season when the Warriors were 41-10 in Curry's 51 games and 17-14 in the 31 games he missed. They were 49-19 with Durant so 9-5 without him (there was some overlap in missed games). Curry's on/off differential was +12.1. Durant was +1.9. And that was fairly consistent all 3 of their seasons together. Curry made the Warriors go even though Durant was the better player. You see it doesn't necessarily equate to skill, talent, etc., merely how integral the player is to the team.
Tatum is far more integral to the success of Boston than Brown is. We've seen this pretty consistently over the last 3 years in that Boston just isn't very good in games Tatum has missed and plays about the same (or better) when Brown is out. It doesn't mean Brown isn't a good player, he absolutely is, but his value to the scoreboard just isn't very high and the reality is, Boston has just been better with him on the bench over the last 2+ seasons, while Tatum has been by far the leader in that category. The fact that they both start and have played similar minutes makes that way more striking as you generally find units to have fairly similar numbers, especially when the individual production has been similar enough. Boston needs Tatum, Boston doesn't need Brown.
What is it in Tatum's game that I'm missing that makes him that great, if we're following your reasoning?
These on/off numbers suggest that Tatum is basically as important as what LeBron James is to the Lakers.
Anthony Davis is a great player, but according (and also to the eye test to be honest) to the numbers the Lakers are only functioning at top level because of LeBron. Same with Harden in Houston in comparison with any other star that played there (Paul, Westbrook).
To me Jayson Tatum looks like 15-20 as a player ranking in the NBA and Jaylen Brown like 25-30. How come this isn't reflected in the numbers?
And will the improvement of Jaylen Brown change this difference between Tatum and Brown?
I have no idea what you are missing. Tatum is that valuable. The wins and losses bear that out. Boston's record when Tatum is out vs. any other player shows this over the last 2+ seasons. Boston is a well below .500 team in the games Tatum misses. That isn't true of any other player (and that includes players like Irving when he was here). And Tatum doesn't even need to play all that well, he just has to play. Boston goes as Tatum goes. Boston needs Tatum.
Lebron James is the most valuable Laker. It isn't close. We saw this on their respective teams before joining up. Davis has never been a player that drives winning teams. Look at his history in New Orleans. That team underperformed a lot during the regular season and as a result had limited playoff success. The one time Davis had a truly elite on/off (11.0) is the one season New Orleans won a playoff series. It isn't a coincidence. Whereas, everywhere Lebron goes they win a lot more games when he is on the court and lose a lot more games when he isn't on the court (or leaves). You can even see it with Lebron, the one season he was really going through the motions for the vast majority of the season, was his last year in Cleveland. He was still +1.9, but that was by far Cleveland's worst season. It was full of turmoil and it was a mess. But in the playoffs, he kicked it up some and was a more respectable +7.5 (and upped his minutes) and the Cavs made the Finals yet again. Lebron for his career is +11.3 and has two ok seasons, that one and his rookie year at 2.2. He had one other season at 7, but otherwise has not been below 8.1 in his career. And his teams win and win a lot because he generally doesn't miss games. The only time he has missed the playoffs since his first couple of seasons, was his first year in LA when he was injured and played in just 55 games (they were 28-27 in those games and 9-18 in the 27 games he missed).
As for Houston, yeah Harden is what made them go. He always has been. Now some of that is how ball dominant Harden is, but some of it is he is just that good and important. Harden didn't miss many games which also helps that out. Even Paul's last year the team was still 14-10 without him, not near the same level, but also not terrible, because they had Harden still.