Mercedes is assessing how it will proceed with its engine plans for the remainder of the season following Valtteri Bottas' engine change in Russia, the second in two races for the Finn.
After exceeding his three-engine quota at Monza, Bottas was assigned a fifth power unit in Russia, which justified the Finn's grid penalty on Sunday.
But a pneumatic problem detected on a Mercedes engine powering Williams' Nicholas also compelled the manufacturer to add a fourth unit to the Canadian's engine inventory.
Championship leader Lewis Hamilton is still relying on two healthy engines, but mileage accumulation coupled with emerging reliability concerns could force Mercedes to add a fourth unit to the Briton's pool of hardware, which would equate to a potentially costly grid penalty for F1's title contender.
In Russia, Red Bull elected to assign a fourth engine to Max Verstappen, which in hindsight proved a timely and massively successful move given the Dutchman's P2 finish in Sochi and which theoretically puts him out of harm's way until the end of the season in terms of engine usage.
©Mercedes
Mercedes is therefore pondering its plans for the remaining six or seven races of the 2021 season as it addresses its recent "gremlins" while also preparing its engine allocations for next year.
"At the moment we are reassessing the performance of the power unit as we have question marks and therefore haven’t decided which engines would go back into the pool," said Mercedes team boss Toto Wolff.
"That's why we're having a few balls in the air, because you need to have the right balance between making sure that you really sort out all the gremlins that you have in the power unit, not only for this year but also for next year's power unit.
"Definitely, we are in a phase of assessment on how to continue the season in terms of power units."
The crux of the matter is that Mercedes, like Red Bull, cannot afford a single retirement in its intense fight for the championship.
"It's always reliability versus performance, it's always a fine line that you need to get right," added Wolff.
"DNFing, obviously, is a no go for the championship and nobody, neither us nor our competitors, can afford a zero points race weekend."
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