Bottas won the previous race in Russia, the Finn's first victory in Formula One.
"In putting the engine back together, we have discovered a small water leak," a team spokesman said, adding that the engine had been replaced with the one used at the previous four rounds of the season.
"We hope that he will be able to run at some point in third practice -- the boys are working flat out on it to make that possible," he added.
The removed power unit will have the leak fixed and will be used at future races and as a spare.
Teams are not allowed to work on cars overnight for an eight-hour period, with two exceptions allowed per season. The governing FIA said Mercedes would not face any sanction since Friday night's breach was their first of the campaign.
Mercedes dominated Friday's free practice with triple champion Lewis Hamilton fastest in both sessions ahead of Bottas.
Ferrari's Sebastian Vettel leads the championship, 13 points clear of Hamilton.
(Reporting by Alan Baldwin, editing by Toby Davis)
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