Arizona Republican Sen. John McCain said Tuesday he is concerned about the looming confirmation process for retired Marine Gen. James Mattis, who President-elect Trump says he will nominate to become the next secretary of defense.

McCain, the chairman of the Senate Armed Services Committee, told reporters in the Capitol he is “very worried” about Mattis’ confirmation process. Mattis will have to receive a waiver allowing him to serve in a civilian position less than seven years after retiring from the military.