The Netherlands international has scored just twice for United, putting him under pressure to retain his spot.
United captain Wayne Rooney returns from a three-game suspension for the Manchester derby, meaning Sunday's clash against Chelsea could be Van Persie's last chance.
Van Gaal said he expected the 31-year-old Van Persie to start scoring again.
"He's a little bit unlucky, like against West Bromwich Albion when he shot with right foot - it was hard to believe that that shot stayed out of the goal," he said.
"So a striker needs also a bit of luck and then the confidence is higher but he's self-confident so I expect he shall score and maybe against Chelsea."
Despite being under an injury cloud, Van Persie scored four goals at the World Cup as the Dutch finished third.
Van Gaal said the striker was fitter now than at Brazil.
"I think he is more fit than ever," he said.
"He don't score so much. In the Dutch team he has scored more than now for Manchester United and then he was not so fit as now.
"That's remarkable, but he scored in [almost] every game for me in the World Cup in spite of maybe his 80 per cent fitness. He scored four goals out of six games and then also a penalty.
"So maybe now he is playing better but doesn't score as much."
Van Gaal talked up the striker's contribution so far this season, and said goals would follow.
"He has showed that at the World Cup and is showing that now except scoring goals but that is only a matter of time," he said.
"Every striker has to be at the attacking point. He has to play with the other 10 players. That is also important. That he is now doing better than in Dutch team."
