Dembele struggled to make an impact during the surprise 1-0 loss.
Calling the game on Dutch TV, Van Halst said: "It was really terrible.
"It was really unbelievable. After fifteen minutes we said: 'He must be put out of his misery'. He lost the ball. There was a bit of discussion because he hasn't played for a while. But you can still play the right way a few times? At one point it was six times in a row. It went on and on. Really a bit sad. There must be something wrong with that boy.
"Seriously, a player with his qualities, a player from Barcelona anyway ... get one from the youth, he has less ball loss than he. It's a big mystery too. I can also imagine (coach Ronald) Koeman thinking: how can it be? How do I get him back to playing football?"