Emilio Alvarez feels David de Gea is not rated as highly in Spain as he is in England because he does not play in La Liga.
The goalkeeper was dropped for Spain's European Champions qualifier against the Faroe Islands on Friday after a disappointing end to last season at Manchester United.
The 28-year-old was at fault for a string of goals, with the Reds' hopes of reaching the semi-final of the Champions League dashed by De Gea's gaffe against Barcelona in the last-eight second leg at the Camp Nou.
De Gea was derided by sections of Spain's support at the World Cup last summer after gifting Portugal a goal in the group stage of the competition, but Alvarez, the United goalkeeping coach, feels Iker Casillas' shoes would be hard for anyone to fill on the national team.
"Whoever was going to follow Iker Casillas for Spain would've found it difficult," Alvarez told Radio MARCA.
"If De Gea played for one of the Spanish clubs, I'm sure that he would've had more support in certain moments.
"Those of us that know him want him to be judged for who he is as a professional and leave it to him to show what kind of person he is."
Kepa Arrizabalaga, the Chelsea goalkeeper, was handed the no.1 spot for Spain's 4-1 win against the Faroe Islands, but Alvarez says it came as no real shock.
"Nothing surprises me in football," he said. "It's a decision that the manager has made and one that we have got to respect."
