Dr. Stack was the coma expert at the Hospital, until he is fired for negligence.
Following a coma caused when Roger is struck by a truck while attempting to photograph his crotch at a red light camera in "Vision: Impossible", Roger has a vision of his Dr. Stack being fired when he touches him while shaking his hand. As the doctor espouses on how good a job he does, the patient in the next bed dies and he realizes the vision came true.
Later, Roger is stuck again and spends another three weeks in a coma. He is told that if Dr. Stack had not been fired, they would have been able to wake him sooner.