There is a trophy/achievement in Thief called "Moral Victory" with the description "Finish the game without a single kill or knockout". Does the tutorial knockout in the prologue not count towards this, or is there a way in the prologue to avoid the knockout?
You cannot proceed through the game without knocking out that one particular guard in the prologue, as you will automatically receive a game over screen if you take too long to take him down.
Because of that, he does not count towards the trophy.
There is one easy way to prove it: Play the game with custom difficulty and select the "No Kills or Knockouts" option. Since you do not get a game over screen after knocking that guard out, you can tell he doesn't count.