Liverpool signed Wirtz in June for a reported fee of 100 million pounds ($135 million), with a further 16 million pounds in potential bonuses.
The 22-year-old had failed to find the net in more than 20 appearances for Liverpool before scoring the winner in Saturday's match, and Slot said his performances had been undervalued due to football's obsession with statistics.
"I'm quite sure it was a relief for him. This I could see after his reaction after he scored the goal – and the same I saw with his teammates. I think they were really happy for him," Slot told reporters.
"In football – rightly so, maybe – we mainly get judged on results, and individuals mainly get judged on goals and assists. Sometimes we tend to forget what else there is to do during a game."
The Dutch manager called on Wirtz to keep going after ending his drought.
"He's had multiple good games for us but I also feel he gets better and better every single game he is playing for us. He gets fitter and fitter and was getting closer and closer to his first goal," he added.
"Then it was not a surprise to me that he scored one today, but he would probably be the first one to understand that one goal is not enough.
"He will score many more goals for us than only this one, but I also liked his performance during large parts of the game today. I think he was special in a lot of moments."
Liverpool, fourth in the standings, next host 16th-placed Leeds United in a league match on January 1.