"It can be difficult to advise the President effectively given his seemingly short attention span and propensity to be easily distracted," a source knowledgeable about McMaster's day-to-day challenges told CNN.

The source added that McMaster's task -- being an honest broker of various national security options for the President -- is further complicated by fears on the National Security Council that Trump can be reckless with sensitive information.

"You can't say what not to say," the source said of Trump, "because that will then be one of the first things he'll say."