The 37-year-old has not yet ruled out going on beyond 2009, telling Voetbal International he will make his decision at the end of the year.
He said: "I must be able to do then what I can do now. If everyone's satisfied, okay. But if you are confronted with injuries and you miss long periods, then you look at it differently."
The former Holland stopper confirmed he had no intention of going back on his decision to bow out of international football.
"When things went so well in the group phase, I was told by the guys that I should reconsider my decision, that I should go on," he said.
"After the games with Italy and France I thought about it for a short while. But after we were put out of Euro 2008 by Russia I immediately thought: this is it, it has been good and that's enough."