Spain coach Vicente del Bosque says the World Cup champion's defense must improve, or even Liechtenstein could prove a stumbling block in European Championship qualifying.
The European champion can clinch its place in next year's tournament in Poland and Ukraine with a victory on Tuesday over Liechtenstein in Group I.
But a poor defensive effort in Friday's 3-2 friendly win against Chile forced Del Bosque to tell his players they may soon pay for those shortcomings if the team's attitude wasn't right.
Del Bosque says "I warned the players - Liechtenstein may be technically inferior to us but they are very well organised."
Spain is without Carles Puyol and Gerard Pique.
Liechtenstein is last in Group I but recently beat Lithuania.