The leak was discovered after qualifying, however, the car then went into parc ferme and the crew left the circuit on Saturday evening not knowing the source of the leak, or how serious it was.
"We had some issues with Lewis's car before the race and this actually became apparent after qualifying," said trackside engineering director Andrew Shovlin in a Mercedes video.
"We dropped the floor, we noticed there was some oil on the floor, so somewhere there was a leak. We could also see on the data we were losing some hydraulic pressure.
"The issue is that the cars are then in parc ferme overnight, we are not allowed to touch them or to investigate it further.
"So, that investigation moved to Sunday morning, when we are allowed to access the cars again, the mechanics can work on them."
It was then that the team confirmed that it faced a serious task, one that involved removing the power unit to replace part of the hydraulic system.
Fortunately, that diagnosis was made early enough to give the team confidence that it could fix it in time, although there was no margin for error.
"We had to take the power unit out. We traced the leak to the throttle actuator, and then we made a request to the FIA to change the throttle actuator and some of the associated hydraulics.
"So, that work was quite a big job though, there's a lot to do within that. And it was pretty tight to get Lewis's car back together for the start of the race.
"Now, we are fairly happy we can do the work in that time. The issue when you have an unknown fault or a leak is you don't know how long it is going to take you to find it, to identify it. We were able to get the car back together.
"Where we were at risk was if we then fired up and we found another problem, if the engine had to come out again, that's when we would've got short on time and perhaps not even been able to start the race.
"But, it was a great job by all the mechanics involved and quite happily the car was back together in time and finished the race successfully."
After the race, the FIA checked that the replacement parts satisfied the rule which says they must be "similar in design, mass, inertia and function to the original" when making changes under parc ferme conditions and deemed it was in compliance with the regulations.