And don't forget that Irving is replacing IT with a potential bum hip. Celtics don't even offer this trade if IT4 is healthy, which he's not or Cleveland would Ta ask for additional compensation.
Even if Isaiah were healthy, I would strongly consider trading one healthy year of him, and Crowder, and Zizic for Tatum. Especially if I had Tatum rated the top prospect of the draft. And especially if I already had Kyrie in hand from trading Fultz for him, Isaiah would be superfluous, healthy or not.