"Williams has its own ambitions and we must always ensure we give our team the best opportunity to move forward," she told the BBC in the first open recognition by either side that a deal could be done.
"Any changes would only be made if Williams remains in a strong position to compete and develop in 2017.
"If we did allow Valtteri to leave, we would only do this if an experienced, credible alternative was available, such as someone like Felipe Massa for example."
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Mercedes, the dominant team in Formula One, are looking for a driver after German Nico Rosberg's shock announcement this month that he was retiring only five days after winning the championship.
Brazilian Massa also retired from Formula One at the end of the season, but that was seen more as a consequence of Williams signing 18-year-old Canadian rookie Lance Stroll to partner the experienced Bottas.
Massa has said he wants to keep racing and might be tempted to return for one more F1 campaign, despite an emotional farewell to his home fans in Brazil last month, to help out Williams.
Both teams have close ties, with Williams powered by Mercedes engines.
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The BBC reported at the weekend that Mercedes team boss Toto Wolff had offered a 10 million euro cut in Williams' engine bill in return for allowing Bottas, the bookmakers' favourite to replace Rosberg, to leave.
Wolff is also a former Williams director, and involved in Bottas's management from a distance.
A Mercedes spokesman said earlier that no announcement was planned between now and Jan. 3 and there was no significance in the latter date, other than it marking the end of the holidays.
Mercedes, who have won both the drivers' and constructors' titles for the past three years in a row as well as 19 of this season's 21 races, have a shortage of experienced and available contenders for the sport's hottest vacancy.
The likes of double world champion Fernando Alonso, at McLaren, and Ferrari's four times title-holder Sebastian Vettel have ruled themselves out.
Mercedes have German reserve Pascal Wehrlein, who raced for Manor this year, under contract but are clearly looking for a more experienced partner for triple world champion Lewis Hamilton.
The sport's commercial supremo Bernie Ecclestone said this week that Hamilton would run away with a fourth championship without a strong team mate.
"I don't think anybody is going to beat Lewis," he told Sky Sports television.
"Nobody would want to buy tickets and go to watch a race or watch on television Lewis disappearing from us when the lights go off, and probably lapping the field a couple of times.
"It would be bad for everybody and bad for Lewis as well, because I think he wants to win fair and wants to beat somebody."
(Reporting by Alan Baldwin, editing by Steve Keating/Toby Davis.)
