"Rafael left early because his calf tensed up which could mean that he pulled it slightly," coach Bert van Marwijk told a news conference.
"He was bothered after Thursday's match (against Cameroon) but yesterday he completed the whole training session, so I can't estimate at this stage if he can play tomorrow."
If Van der Vaart is not fit enough, Van Marwijk might consider starting winger Arjen Robben, who missed the first two matches of the campaign with a hamstring strain.
The Dutch team's most potent attacking weapon, Robben made an immediate impact as a late substitute in their final Group E match against Cameroon when his shot hit the post and striker Klaas-Jan Huntelaar netted the rebound to secure a 2-1 win.
"Arjen felt no reaction to his hamstring after the Cameroon match and I speak every day with him but in the end it will be my decision (whether he starts)," Van Marwijk added.
NO REPEAT
The Dutch are overwhelming favourites to beat the Slovaks, who sent holders Italy home with a sensational 3-2 upset win on Thursday, but Van Marwijk said the squad had practised penalties just in case.
The Dutch coach added he had been drumming it into his players that there should be no repeat of the lapse of concentration they suffered after taking the lead against Cameroon, which allowed the Africans to equalise.
"You see many teams having a poor spell after they have taken the lead, but we suffer from that more than most," Van Marwijk said.
"We seem to believe that we can sit on the lead, but we cannot afford to do that, especially not in this phase of the competition.
"We did it the first two matches and stayed in control but against Cameroon for the first time I had the feeling we could have lost," he added referring to their 2-0 opening win over Denmark and 1-0 victory against Japan.
