"I hope it's not the end of my weekend before it's started," the Briton told reporters on Thursday. "I am still confident we can have a good weekend."
Norris started last year's race at Spa-Francorchamps on pole position, with teammate Oscar Piastri winning, but McLaren have been off the pace this season and are third overall behind leaders Mercedes and Ferrari.
Drivers are allowed three power electronics units per season and the change exceeds the limit, triggering the mandatory penalty.
Terminal failure in China, issues in Japan
McLaren said the first unit suffered a terminal failure in China in March, leaving Norris unable to start that race.
The second unit was fitted for the Japanese Grand Prix, also in March, but had to be withdrawn for remedial work after issues in free practice.
It was repaired but then suffered a terminal issue in Monaco practice in June.
"While the power electronics unit we installed in Japan, and have used in every session since Miami, has worked reliably, Mercedes-AMG High Performance Powertrains has since introduced a series of reliability fixes," the team said.
"However, in order to take advantage of these improvements, we must incur a 10-place grid penalty on Lando's car in order to take a new unit.
"We have chosen to do this in Belgium, a circuit where overtaking is relatively more prevalent, as opposed to the following two events in Hungary and Zandvoort (Netherlands)," the team added.
McLaren said they planned to use this fourth unit for the remainder of the season.
“It won’t be made easier by the grid penalty but that is because we have been unlucky losing different bits, the engine, the power unit or the controls," said Norris, who has a Belgian mother, of Sunday's race.
"I am on the back foot in terms of a spare parts point of view but that is out of my hands. That’s life, you’ve got to take it on the chin. This is a better track to take the penalty than the next two."
Piastri will have the latest Mercedes engine without penalty due to not suffering the same issues on his car.
McLaren are the last of four teams with Mercedes engines to get the latest edition, due to their own schedule.