Deputy Attorney General Rod Rosenstein. (Getty Images.)

Commenting on the resignation of U.S. Attorney General Jeff Sessions, former federal prosecutor Joe diGenova said Sessions' departure was long overdue and added that the "days are numbered" for Deputy Attorney General Rod Rosenstein, the Justice Department official who ordered the appointment of a special counsel to investigate alleged "collusion" between Russia and the 2016 Trump presidential campaign.

In a Nov. 8 interview on WMAL's Mornings on the Mall in Washington, D.C., diGenova, a former U.S. Attorney for the District of Columbia, said, "I can tell you the person who won’t be attorney general and that’s Rod Rosenstein."

"In a little related event, when Rod Rosenstein went to see General Kelly [Trump’s chief of staff] for his regular meeting [on Wednesday]," said diGenova, "he was told to cancel his speech at the Aspen Institute today on the West Coast."

"I think Mr. Rosenstein’s days are numbered," he said.

When asked how soon Rosenstein might leave the Justice Department, diGenova said, “I would think relatively shortly because Mr. Whitaker," the new acting attorney general, "has taken over control and supervision of the Mueller thing," the special counsel investigation of alleged collusion.

"So, Mr. Rosenstein’s usefulness as an overseer has now evaporated," said diGenova. "That was a strategic move to strip Rosenstein of his responsibilities. If he doesn’t quit, I think he’ll be fired.”