The Finn was set to take a grid drop after damaging his gearbox when he crashed out in the early moments of Q3 after losing control in the Parabolica.
However, Alfa Romeo subsequently decided to change his internal combustion engine, switching back to the previous spec, and control electronics, and therefore the Finn will start from the pitlane.
The Mexican suffered an engine issue during qualifying, the second successive weekend that he has been sidelined by Mercedes latest-spec unit.
As a result, Perez is reverting to the Spec-2 version.
"If we ever go to the spec-3 then it is a trade-off between more performance and penalties," said team boss, Otmar Szafnauer, last night. "But, heaven forbid, say one time we spin off in Q1 or something fails in Q1 and you are last anyway, then you can introduce the Phase 3."
Asked if reliability concerns over the latest-spec Mercedes were also behind the decision to stick with the spec-2 - Robert Kubica also having suffered a failure with the new unit in Belgium, Szafnauer said: "I don't know enough, need to ask Mercedes.
"I don't know if there is this infantile failure that you get in some products, like electronics, where if it doesn't fail in the beginning, the probability of it failing goes really, really low. But the probability of failure at the beginning is higher. But that is one for Mercedes.
"However, they are really, really good at reliability in general and they will get on top of it. They are really strong on that."