The Saints lie in second place in the table after five games, but the Dutchman said that Champions League ambitions are unrealistic at this stage.
"It isn't realistic" he told reporters. "We have played only five games in the Premier League and at the end of the season are 38.
"We know it's a long season, it's a tough season. Okay, we know we have a very good start and, of course, after the win of last Tuesday, the expectations of the fans and of the press is much higher than it was before the first game.
"But it will be more difficult and we have to realise that and to keep going."
Southampton play host to Queens Park Rangers this weekend.