From the guidelines reference

You can carry out the evade motion solely if you’re engaged with a number of monsters.

And from Secrets of the Order Expansion Rules the instance given says

During the monster section, the Raging Poltergeist strikes into Agatha’s area and engages her (2). The monster card flips to the engaged aspect and turns into a Confounding Specter, revealing that its well being is 2.

You cannot use evade with out being engaged to a creature, however as soon as they’re engaged you reveal the aspect of the cardboard with the statistics. Once an investigator is engaged to a creature, they will use the evade motion.