German Wehrlein, 22, is the senior member of Mercedes's driver development programme and was therefore a favourite when Nico Rosberg announced his shock decision to quit F1 late last year.
"Toto told me he didn't want to burn me," Wehrlein told Germany's Sport Bild.
"He said they still believe in me and that I should instead build myself up in peace. Of course I hoped for a change to the world champion team, but when Mercedes went for Bottas, I wasn't too disappointed.
"Why? Because my original 'plan A' was still fulfilled."
That 'plan A' was the 2017 move from the backmarker Manor to Sauber, a more established midfield team.
However, winter testing has shown that, after almost collapsing last year and now set to use Ferrari's 2016-spec engine this year, Sauber will almost certainly be F1's new backmarker in 2017.
Still, Wehrlein is happy, saying: "Sauber is at the beginning of an era. There is enormous potential in the team and I am proud to be a part of this building back up. We have a highly professional, international team."
For instance, Wehrlein said that he was blown away by Sauber's state of the art wind tunnel at Hinwil, adding: "As I said, I think there is huge potential in the team.
"But we will have to have a little patience."
The 2017 championship kicks off with the Australian Grand Prix next Sunday.