To increase space, doctors at Northwestern Memorial Hospital removed two small pieces of tissue previously destroyed by the stroke.
Northwestern officials were unable to provide further details.
Kirk had emergency surgery Sunday after going to a hospital with stroke symptoms. At that time, Fessler removed a piece of Kirk's skull to alleviate pressure from swelling.
The stroke affected Kirk's left side, particularly movement of his left arm, leg and his face. Doctors say he is expected to make a full mental recovery but might not regain full movement, even after rehabilitation.