For the second race in a row, Mercedes took the precautionary measure of installing a new internal combustion engine, as well as fresh MGU-H and turbocharger components, on Valtteri Bottas’ car at Sochi in what was described as being a “tactical” move.

This was initially seen as an attempt from Mercedes to disrupt Max Verstappen’s progress through the field but trackside engineering director Andrew Shovlin revealed after Sunday’s race the decision was taken because the power unit introduced at Monza had developed a problem that needs further investigation.

After facing grid penalties at back-to-back races in Italy and Russia, Mercedes boss Toto Wolff confirmed the team is “reassessing” the performance of its engine.

“We haven’t only made the precautionary engine change because we want to stockpile, but also because we wanted to understand the engine’s performance,” Wolff explained.

“At the moment we are reassessing the performance of the power units  because we have question marks and therefore haven’t decided which engines would go back into the pool.”

Bottas’ issue is the latest in several engine-related concerns for Mercedes over the course of the 2021 season which have not just affected the works team.

A pneumatic problem discovered on Nicholas Latifi’s car on Friday forced Williams to fit a new Mercedes engine and trigger a grid drop, while McLaren also found an issue on Daniel Ricciardo’s power unit during first practice.

Ricciardo avoided a penalty for the race after McLaren swapped his engine with one from his existing pool.

The uncertainty is a particular concern given that Mercedes is currently working on its 2022 power unit for homologation with an engine freeze coming into effect next year.

“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,” Wolff acknowledged.

“Definitely we are in a phase of assessment of how to continue the season in terms of power units.”

Asked how concerned he is about the prospect of a potentially title-defining DNF during the final seven races of the season, Wolff replied: “Reliability versus performance is always the fine line that you need to get right. 

“DNFing is obviously a no-go for the championship and nobody, us or indeed our competitors, can afford a zero-point race weekend.”

Red Bull got their anticipated engine penalty out of the way in Russia as Verstappen limited the damage by recovering from the back of the grid to claim an excellent second place behind main rival Lewis Hamilton.

Although Hamilton finally scored his 100th grand prix victory to move back into a slender championship lead, the Mercedes driver is likely to have to take his own engine-related grid penalty before the end of the season.

Hamilton has just two used engines left in his pool to take him through the remaining seven rounds after the seven-time world champion lost an older, high-mileage PU following a stoppage during practice at Zandvoort.

“I’ve lost one engine, Valtteri’s had several,” Hamilton said.

“And there’s been others that Mercedes have seen up and down the paddock.

“So I’m trying to treat my engines with the absolute care when I’m driving, in terms of how much I’m gassing it, in terms of revving the nuts off the thing, just really trying to minimise the laps that I do.

“But who knows? I can’t control the future.”