The biggest thing Jaylen haters point to is the advanced metrics - specifically the on-off stats. Jaylen's on-off stats haven't been great his whole career - like he's only been in the top 50% of wings 3 seasons out of 10, and he's been in the bottom 20% percentile 3 of the past 4 seasons, including his favorite season last season when he was in the bottom 24th %ile.
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If you compare him to Tatum, Tatum's on off metrics are much better, even last season when he only played 16 games he was in the top 68th %ile of wings in on-off metrics.
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And the salary dump we are getting, PG0 (I mean PG13) has much better on-off metrics:
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This might be clutching at straws to defend the indefensible, but those advanced metrics are probably something that the Celtics took into account in making the trade - but they probably weren't the be-all end-all, it was probably that, along with a possible relationship breakdown between JB and the front office, and a possible alpha dog conflict between JT and JB this season, which might have brought back unpleasant memories of 2018-19, along with a reluctance to extend JB to another supermax, which brought about this trade.