Netherlands coach Guus Hiddink criticised his defenders after a series of errors led to a 3-2 loss to Mexico on Wednesday in Amsterdam.
The defeat was the team's fourth in five games since Hiddink took over a squad that finished third at the World Cup under Louis Van Gaal.
The Dutch have conceded first in all of their games since they returned from Brazil.
On Wednesday, Carlos Vela was given ample time and space in the middle by centre-back Joel Veltman to curl a left-foot shot into the top corner of Tim Krul's goal.
"You have to learn very fast that you can't do this at international level," Hiddink said after the game of his team's defensive lapses. "You must not make these mistakes internationally. We have trouble defending. It all went in fits and starts.
"On that first goal, everyone was too remote. We did not dive into the game. Then a series of errors followed."
The Netherlands defence was dealt an added blow when centre-back Ron Vlaar limped off the pitch in the 24th minute and was replaced by the younger Stefan de Vrij.
Hiddink also watched as his team missed some chances to score, with Klaas-Jan Huntelaar missing some particularly golden opportunities.
"We showed a lot of power, only we were not conclusive. In the final third, if you get seven or eight chances, you should make at least three."
Captain Arjen Robben agreed that Netherlands' struggles don't all rest on their defence.
"It's clear we're vulnerable,'' Robben said. "Up front, we have to score more goals."
Hiddink said last week that he would resign if his team lost to Latvia in Sunday's European qualifier, and he said the defeat to Mexico did not add any extra pressure.
"That pressure is always there, but every defeat is a blow," he said. "If we make the same errors [against Latvia] that game will also be difficult."