With both teams unwilling to give away space in defence at the Estadio Nacional de Costa Rica, the friendly lacked genuine goal-scoring opportunities and deservedly finished 0-0.
Shots from outside the area were the order of day, while the home side's Joel Campbell and Paraguay's Raul Bobadilla were the most lively attackers for either team.
Speaking after the game, Costa Rica coach Paulo Wanchope claimed "both teams did a great job on defence".
Diaz was chosen to replace caretaker coach Victor Genes in December, after previous boss Gerardo Pelusso had departed during qualifying for the 2014 World Cup.
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