But van Persie believes the preparatory work the squad are doing with boss Louis Van Gaal - who will manage the striker at Old Trafford next season - will leave them well placed to succeed.
"Normally you have more time to develop another system, now we have to make a little step every day. We are in a bit of hurry," he told Voetbal International.
"We only have a few weeks and two friendlies before we play against the best team in the world. In my opinion, Spain is still the best.
"I will not discuss our system too much, because our opponents could benefit from that. They just have to wait on how to deal with it.
"I really believe in this system. When you see how much progress we make every day, that's very positive. I'm not only talking about the training, but also our meetings off the pitch.
"The coach already said that we're taking risks. If every players fullfills his task, this system can be fantastic.
"Within a few days I saw so much progress in our team. We just know what the coach means and that's very nice to see. We really have to connect."
"Last week we had a very fine moment at the training, where all the players and staff were on the same level. Everybody understood what we have to do," he added.
Netherlands will also face Australia and Chile in an tough-looking Group B.