Felipe Massa has suggested that he is unhappy with the Williams team.
Earlier at Suzuka, team official Paddy Lowe said that while the Brazilian veteran is still in the running for 2018, Robert Kubica and Paul di Resta will be tested "in the next few weeks".
Another rumour is that Pascal Wehrlein could get Massa's seat, perhaps as part of a discount on Williams's expensive Mercedes engine bill.
Massa said in Japan: "I'm concerned that the team could choose a path that could bring difficulties for the team.
"I think Lance [Stroll] has made a lot of progress through the year so perhaps Williams should keep the current lineup.
When asked about Lowe's admission that plenty of candidates are under consideration for the seat alongside Stroll next year, Massa said: "Paddy knows what's best for the team.
"Unfortunately, this is just Formula 1. I think if Williams acts professionally, they will leave everything as it is."
Massa, 36, actually retired at the end of last year, and only returned to Williams when Valtteri Bottas headed to Mercedes at the last moment.
Asked why he is now so keen to stay in F1 for another year, Massa explained: "In the past three years the cars did not suit my driving style.
"There was not much grip, the tyres did not work in a good way - it was not fun and I could not compensate with my style.
"Now, when I can feel that I can push to the maximum, the motivation increases. I still believe I can give a lot to the team."
The 2017 season continues with the United States Grand Prix in a fortnight's time.
