Same as with Rozier. It's one thing to perform and another to perform when they game plan against you specifically.
Now, one thing about Tatum is that he does everything within the flow of the offense. Which may be good or bad for individual performances. Bad because you'll have inconsistent opportunities, good because it's hard to gameplan against. Also, he's not as good as Hayward creating opportunities for others.
That said, Hayward is very good, often underrated, but he's not that mega superstar. It's not far-fetched to believe that Tatum can gain on and surpass Hayward quite quickly. I'd also say that Hayward will benefit greatly from the way the Celtics play vs how he did things in Utah, though he had an incredible last year in Utah.
Worth mentioning that Hayward also greatly transformed his body to be as effective as he's now (pre-injury). Let's see how much Tatum can transform his body, and how it'll affect his game (which there's a risk of going either way, but leaning towards improvement).
Let's not forget that Tatum is a mismatch for Philadelphia. Though he's probably a mismatch for most teams as it is.
He's been impressive.