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After the hard foul by Williams on Rondo, Posey took two steps on to the court and jumped back.

Can Posey be suspended for stepping on the court after a hard foul or does there have to be pushing and shoving for him to be suspended for a game?

After Williams flagrant two'ed Rondo, there was no shoving or jawing between the players, so Posey can't be suspended. Just another foul and another player stepping on the court like they always do.

What is the official rule on this?

No there are no fight after the foul so the rule does not apply

There needs to be an "altercation", and that didn't happen. Almost the exact same thing happened with Tim Duncan (I think last year) and there wasn't a suspension.

No. You don't get suspended for taking 1 step on the court, you get suspended from going way off the bench and running to a fight during it like Amare and Diaw did.