Pascal Wehrlein and Esteban Ocon will share the role of Mercedes's reserve driver in 2016.
The German has now secured his Formula 1 race debut for 2016 with Manor, but Mercedes team boss Toto Wolff said that 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," he told Auto Motor und Sport.
"So in this case we would not want to pull Pascal out of there overnight. If the thing happened at very short notice, Esteban Ocon would be an alternative for us."
Reigning GP3 champion Ocon, 19, will be the third driver at the Renault works team as well as co-reserve driver at Mercedes alongside Wehrlein.