Pascal Wehrlein and Esteban Ocon will share the role as Mercedes’ reserve driver in 2016.
Last year, as he charged towards the DTM title, it was Wehrlein who would have stepped in had Lewis Hamilton or Nico Rosberg been unable to race through illness or injury.
But the German has secured his F1 race debut for 2016, with Manor.
Still, Mercedes team boss Toto Wolff said it is possible Wehrlein will be available to step up to the main team if required.
“If it happens a lot in advance, then we would find a solution with Pascal and Manor. But it is important that Manor is not damaged through this.
“So in this case we would not want to pull Pascal out of there overnight,” he told Auto Motor und Sport.
“If the thing happened at very short notice, Esteban Ocon would be an alternative for us,” Wolff added.
Frenchman Ocon, 19, is the reigning GP3 champion who is now hot on Wehrlein’s heels as the cream of Mercedes’ development programme.
His main role in 2016 will be replacing Wehrlein at DTM, but he will also be the third driver at the Renault works team.
And a third role will be as co-reserve driver at Mercedes alongside Wehrlein, Wolff said.
“He (Ocon) knows the Mercedes simulator, and we have already made a seat for him,” said the Austrian. “In that case we would surely reach an agreement with Renault.”