Mercedes said Hamilton, a five-times winner at Monza, would be using his fourth power unit after damaging one at the Belgian Grand Prix when he collided with Alpine's Fernando Alonso on the opening lap.
"This will come with associated grid penalties," a spokesman said.
Mercedes said after Spa that Hamilton's Mercedes took a 45G vertical hit when the car pitched high into the air before coming down hard on the asphalt runoff area at Les Combes.
Hamilton then retired from the race with a loss of water pressure.
A team spokesman said they were working on a recovery plan for that power unit but it could not be run this weekend so a new one was needed.
Hamilton is sixth in the championship but 152 points behind Red Bull's Max Verstappen with seven races remaining.
