Felipe Nasr's uncle and manager insists the Brazilian is the "real third driver" at Williams this year.
The comments made by Amir Nasr, a well-known figure in the world of Brazilian motor racing, follow news Susie Wolff will drive the new Williams on Friday at both the British and German grands prix this year.
Just days earlier, the Grove based team appointed the Banco do Brasil-backed 21-year-old Felipe Nasr to the test and reserve role, saying he will appear in the Mercedes-powered FW36 on five Friday mornings in 2014.
Speaking to Jovem Pan radio, Amir Nasr played down suggestions Felipe's new role could be a shared or back-seat one, such as when Lotus reserve Davide Valsecchi last year was overlooked when Kimi Raikkonen headed for back surgery.
"Felipe is the real third driver of the team -- it is in the contract," Amir Nasr insisted.
"In case of any problem with the race drivers, he is the natural replacement.
"He will have to go to all of the grands prix and he will work in the simulator each week to compensate for the lack of track time.
"It is a more complex task than it seems," Amir added.
Amir Nasr said only a one-year contract with Williams was agreed because the chance the British team can offer him a race seat for 2015 is slim.
"Williams has two very good (race) drivers," he said. "Massa has a three-year contract while Bottas is 23 and a potential world champion.
"We could not agree a longer contract because we would not have a guarantee he will be promoted," Amir explained.