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.