I'd be surprised if UE starts with Malen in the side: he has shown precious little faith in him so far. And, as you say, Onana gives us more physical presence.
Last season though it was in the context of fitting Asensio and Rashford in though, and I'd say that Malen has been one of the stronger performers in attack in pre-season.
And I dont think he's going to throw Guessand straight in.