Valtteri Bottas inherited a plum drive at Mercedes after Nico Rosberg's retirement but the Finn will need to deliver the goods quickly, says Toto Wolff.
As Bottas and team Lewis Hamilton enter their last day of pre-season testing, the pair appear to be working well together at Mercedes.
Mercedes AMG F1 non-executive chairman Niki Lauda is pleased with how Bottas has settled in with the German squad.
The new recruit is still finding his marks and remains outpaced by Hamilton for now, both on the short and long runs, according to Lauda.
Like his colleague and countryman, team boss Toto Wolff is giving Bottas four races to unleash his speed.
"Valtteri does not have a lot of time to prove he can be at Nico's level, to perform well against Lewis Hamilton and help the team keep up the development direction and speed in the car," Wolff said.
"It is clear that Valtteri needs to learn how to swim quickly."