As Siewerts team were defeated 4-0 at the Tottenham Hotspur Stadium, regular starter Mooy was left on the bench to the surprise of many, making a substitute appearance in the 82nd minute.
As quoted by Examiner Live however, Siewert supported his decision and said that Mooy offered a clearer threat coming off the bench.
"For me, its important to talk about what he did when he came on the pitch.
"He directly gave everything."
Having scored three times this season, and with Huddersfield already relegated from the top flight, perhaps Siewert simply wanted to give some other players an opportunity.
"Aaron is a fantastic player, a really fantastic player, but we had another idea and he knows that.
"He is a great guy and he directly helped that we had our chances."
As the scoreline reflects however, Siewert had to make frequent changes throughout Saturdays game as his team struggled to create chances.
Adding to his explanation, the German said: "When Tottenham changed their shape we had to be intelligent and look at what was possible.
"We brought Jon [Stankovic] a bit forward and changed to 4-3-3, and they also changed, and we directly had possibilities.
"Each player who came on the pitch responded with what I want from my team."
Huddersfield are currently rooted to the bottom of the Premier League table with 14 points, and with their relegation already confirmed, Terriers fans may start to fear the looming transfer window as clubs look to entice Mooy away from the John Smiths stadium.