Valtteri Bottas has reportedly lost the fourth engine from his pool for this season, after Mercedes uncovered problems with the unit back at their engine headquarters in Brixworth.
Bottas introduced a fourth power unit at last month's Italian Grand Prix, triggering an engine grid penalty as just three units are permitted for the season.
Using that engine for the Monza race, it was removed early during the Russian Grand Prix weekend as Mercedes opted to fit another fresh unit to Bottas' car in Sochi. This triggered yet another grid penalty.
There were whispers that this was a tactical move to put Bottas further back and try to hamper Max Verstappen's recovery from his own engine grid penalty, but this doesn't appear to have any validity to it, if the latest reports are to be believed.
Germany's Auto Motor und Sport report that Mercedes took Bottas' engine, as well as their entire stock of current power units, back to Brixworth for extensive analysis. Following this investigation, Bottas' power unit from Monza was ruled out from use in a race again, having completed just one Grand Prix distance.
The issues are believed to be linked to the Internal Combustion Engine itself, the part of the power unit that Hamilton has replaced for this weekend's Turkish Grand Prix. He will serve a 10-place grid penalty as a result.
The other hybrid parts are understood to not be a concern in terms of reliability, although it's meant some extra work back at base to prepare Hamilton's power unit for this weekend, as it's particularly unusual to mix used ancillaries with a completely new engine.
"This is a job that you only want to do in the factory because it is so complex," Auto Motor und Sport quote a Mercedes engineer as saying.
"We have only mixed components from different drive units on the track once in the past seven years."
When approached for comment by RacingNews365.com, Mercedes elected to neither confirm nor deny the story, instead declining to comment. For reference, Mercedes did confirm a prior engine issue for Lewis Hamilton at Zandvoort, when approached for comment by this publication.
Lewis Hamilton claimed his 100th Grand Prix win to move into the championship lead, but it's Red Bull and Max Verstappen who will be happiest after the Russian Grand Prix.