Mercedes is facing a race against time to change the engine in Valtteri Bottas' W12 ahead of qualifying in Saudi Arabia after detecting a fuel leak in final practice.
Bottas was only sixth quickest behind F1 championship leader Max Verstappen in the Saturday session at the Jeddah Corniche Circuit.
But with less than an hour remaining before the vital qualifying session, Mercedes confirmed it was up against it to switch over to an older unit.
A Mercedes spokesperson said: "We had a fuel leak on VB's PU at the end of FP3.
"Without sufficient time to diagnose and resolve, we are swapping it for a unit from the pool."
As the change is for an older engine it will not result in a grid penalty for the Finn.