Mr Netanyahu has described the comments as totally unacceptable. He is due to visit the White House next week for a US-Israeli summit. Israeli officials say Mr Baratz will not be accompanying him.
US state department spokesman John Kirby said Mr Baratz`s Facebook posts were "troubling and offensive".
He said: "We obviously expect government officials from any country, especially our closest allies, to speak respectfully and truthfully about senior US government officials."
"The secretary spoke with Prime Minister Netanyahu this morning and we understand that the prime minister will be reviewing this appointment when he returns from his visit to the United States."
Rivlin `marginal`
The Facebook posts emerged shortly after Mr Netanyahu announced the appointment of philosophy lecturer Mr Baratz as his chief spokesman on Wednesday.
In March, Mr Baratz described President Obama`s criticism of Mr Netanyahu`s opposition to the Iran nuclear deal as "the modern face of anti-Semitism in Western and liberal countries".
He was also critical of Israeli President Reuven Rivlin, saying he was "such a marginal figure" that even Islamic State militants would not want him as a hostage.
Mr Netanyahu quickly distanced himself from the comments.
"I have just read Dr Ran Baratz`s posts on the internet, including those relating to the president of the state of Israel, the president of the United States and other public figures in Israel and the United States," he said in a statement.
"Those posts are totally unacceptable and in no way reflect my positions or the policies of the government of Israel. Dr Baratz has apologised and has asked to meet me to clarify the matter following my return to Israel."
Mr Baratz`s appointment is still subject to cabinet approval and some ministers have said they will oppose it.
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