But you can put someone different in, if not a someone who'd already been in the game. You're largely not required to have Player X for subsequent plays in if he's melting down.
You don't usually hear this exchange in the stands:
"Get that guy outta there!"
"He can't until two more batters..."
Right, which is why it would be a change. I'm just pointing out that baseball isn't exactly a world of free substitutions. Maybe the only major sport with more strict subtitutions is soccer?