Heimerdinger was originally scheduled to begin treatment for the undisclosed form of cancer on Monday, but Fisher said it was being postponed "for good reasons."
Heimerdinger is expected to help put together the game plan for Sunday's contest against the Jacksonville Jaguars.
"He will be gone the entire day Wednesday and will probably be away from the building for several hours on Thursday and Friday," Fisher said. "We'll see how he feels towards the weekend as far as how the play-calling duties will unfold."
Fisher said there is a possibility that Heimerdinger could call plays Sunday.