The Terriers were undone by a brace from Gerard Deulofeu. Karlan Grant struck for Huddersfield.
Siewert lamented, “Unfortunately after five minutes we were directly on the back foot, but still there were very positive things for me to see today.
“The result of course is very painful, but Watford is a good and organised team and we really could've hurt them today.
“I'm proud of how the lads tried to be on the front foot in my system."
Siewert's side created a lot of chances during this afternoon's game and he was pleased with the players' performances in his preferred 4-3-3 system.
“I learn so much from these games because the chances will also be there in the Championship and you can in your way then punish it, for me it's important to go through this process.
“It belongs in football to have moments where you're disappointed but it's about being positive and changing it.
“I can tell the fans and all the supporters that they must stay positive as I know what to do."