Speaking exclusively to Football Presse, Van Loen said he was surprised Liverpool chose to part company with Slot, arguing injuries and tragic circumstances should have been taken into account rather than judging the manager solely on results.
"I think it is a very rare decision," Van Loen said. "Last year he was champion of England and now they sacked him because they did not get the results that were enough to win the championship.
"But he had a lot of injuries.
"And then there was the death of his player (Diogo Jota).
"I think it is not the right decision or the good decision for Liverpool."
Van Loen believes Slot remains one of Europe's leading coaches and is convinced the former Liverpool boss would have continued to improve if he had been given more time.
"I think they must keep him because he is a very good coach," he continued. "I know for sure he will become better and better.
"That is why I think it is the wrong decision."
The comments come after the Netherlands' World Cup campaign ended in a surprise penalty shootout defeat to Morocco, with Ronald Koeman stepping down as national team coach in the aftermath. Van Loen admitted he had been encouraged by much of what he saw from the Oranje despite the disappointing exit.
"I think they played well," he said. "They only have some bad decisions in the back line.
"But I think they will get better and better."
The former striker also believes the overall standard of the World Cup has improved as the tournament has progressed.
"In the first couple of games it was not good," he said. "But then the second games were better.
"And now the games are even better because teams have to win or draw. You see better football.
"You see physical games.
"You see technical quality at a high level."
Among the standout nations, Van Loen singled out France as the team that had impressed him most.
"I think France are very, very good," he said. "Spain is very good."
However, he admitted he hopes one of the traditional favourites falls short of lifting the trophy.
"I hope Argentina cannot win the World Cup," Van Loen said.
He also questioned some of the officiating involving the South Americans during the competition.
"(Lionel) Messi should have had a red card in the first game. There were also decisions from the referees."
Away from international football, Van Loen believes PSV Eindhoven remain the team to beat in the Eredivisie despite the traditional challenges posed by Ajax and Feyenoord.
"Normally it is PSV, Ajax and Feyenoord," he said. "But I think PSV Eindhoven will become champions.
"They have a good coach in Peter Bosz.
"Ajax are rebuilding a new team, so they are going to lose a lot of points.
"And Feyenoord are not the team that can win the championship."
Van Loen also revealed he would relish the opportunity to return to the dugout if the right role became available. The former Dutch international has built up extensive coaching experience since ending his playing career, working in youth development and as an assistant at FC Utrecht before taking charge of clubs in the Netherlands and Georgia.
A UEFA Pro Licence holder, he believes he still has plenty to offer the game.
"If clubs are interested, they can call me," he said. "I hope to get a good team, whether as an assistant coach or a head coach.
"If there are clubs in Europe or somewhere else that want me, they can call."