Pioli has slammed the online abuse aimed at the USA captain.
Ahead of their Europa League clash with Slavia Prague, he said: "He must be defended and protected: history speaks for him.
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"Social media is now an outlet for rudeness and insults: Christian has all our support. But he is too much higher than the others."
On Rafael Leao's European form, Pioli added: "We need to hit the same button: Rafa has the qualities to be decisive in every match and even when he didn't score he was decisive.
"He must think about being decisive in every match."