Can NOT Use a Deserted Knight, as Per Official Rules
Just to clarify what the official rules states on the FAQ page:
Question about “Cities & Knights” Expansion
Progress Cards: Deserter - May I immediately use a deserted knight to perform a knight action?
Yes, provided that he was activated when he deserted to you.
The game (at least on Android) DOES NOT work this way.... If you steal a knight, and place it on an intersection adjacent to the robber. EVEN WHEN IT WAS ACTIVE AS IT DESERTED TO YOU, the game will not allow you to use the recently-deserted knight to "Chase Away the Robber."
There's no screen capture, but honestly.... It's not like it's difficult for you to test this for yourself.