There isn't any mechanism by which a party leader can actually force anyone to step down as an MP. They can kick them out of the caucus/party, but since an MP has been directly elected by their constituents, they can't forcibly take away their seat.
So usually what they have to do is find someone who:
A) Believes in them as a party leader.
B) Believes in their ability to deliver future rewards or career advancement if they do this big favour for them.
C) Is OK with stepping away from their seat.
...and then if Pierre does find that unicorn, he can also bribe them in the short-term with a well-paying job with the party, or some affiliated Conservative organization or think tank. Pretty uncharted waters for a Federal leader to try this after losing the election and their seat, though. So it'll be interesting both to see if Pierre can find that person, how long it takes him to convince them to step down and then how soon Carney calls a bye-election to fill the vacancy.
The only other similar situation I can think of is John Tory losing the election and his seat with the Ontario PC's. Then he stayed on as Party leader for more than a year, I think, before he convinced another MPP to step aside for him. And then he promptly lost that bye-election too before finally stepping down as party leader. But I assume Pierre will try to find an ultra-safe seat in Alberta to avoid that second possibility.