On/Off differential shouldn't be used to rate a given players worth to the team without a ton of context being considered as well. Tatum is most often pulled near the 6 minute mark of the 1st quarter and then comes back to play with the bench a few minutes later. The players off the C's bench get a tremendous boost from this and their numbers would likely not be anywhere near what they are without this strategy being deployed. This same strategy is used by many other teams with star players such as Tatum. Utah is another example with Markkanen used similarly, which bolsters their bench greatly.
Jaylen largely plays starter minutes, 1st 9 minutes of the 1st and last 9 of the 2nd quarter. Jaylen rarely gets opportunities against the weakest lineups other teams deploy. If you reverse Tatum and Jaylen's usage and brought Brown back with the bench or if you played them both together starter minutes you would see completely different On/Off differentials.
On/Off differential like all basketball statistics must be viewed with a ton of context or you can very easily be led astray. A players given role, the players they play with, a players quality of replacement when they sit and many other factors contribute to their success or failure on the floor. Brown is not Tatum and cannot carry the team in the way he does, just as Kyrie was to Lebron, Pippen was to Jordan etc.. He shouldn't be slighted because he cannot, very few players could fill those shoes and show the same level of success.
Jaylen has played 545 minutes this year so far without his star teammate on the floor with him, in those minutes the C's are +21. The entire team dynamic changes for a few short minutes when Tatum sits, usually against opposing starters, and the C's probably wouldn't be winning those minutes without Brown.