Pascal Wehrlein could be set to lose the backing of Mercedes.
If Sauber and Williams say no, that could be the end of German 23-year-old Wehrlein's Mercedes backing in F1.
"Williams is the only option that remains," German language reports quote Mercedes team boss Toto Wolff as saying when asked about Wehrlein.
"They are considering several drivers. Pascal has a chance but we can do no more than what we have done.
"At some point a driver needs to stand on his own legs."
"If we are talking about Pascal, then we do not need to test him because we see him drive in every race," he said.
Lowe, however, admitted that Williams is having to consider the wishes of main sponsor Martini, who would prefer that only drivers over 25 advertise their alcoholic drinks.
"Yes," Lowe confirmed, "but I'm also sure Martini realise how important the drivers are to us.
"We will choose the option that is best in terms of results, rather than the one that works best for the sponsors."
Another rumoured option for Wehrlein, the Red Bull junior team Toro Rosso, has been ruled out.
"It's nothing to do with Wehrlein personally," said Dr Helmut Marko. "We are just not somewhere for Mercedes juniors."
Wehrlein has picked up five points for Sauber this year.