The 27-year-old attacker broke the world record transfer fee in 2009, when he joined Madrid from Manchester United in a €94 million deal.
He has, however, proved value for money at the Santiago Bernabeu, with his double against CSKA Moscow on Wednesday taking his tally to 128 goals in just as many appearances for Los Blancos.
"I have never seen anyone else play at his level," Casillas is quoted by Marca as saying. "His stats are incredible.
"The truth is that I never thought that they would be this impressive this soon and that he would take such a big role in the team.
"He has surpassed the likes of Brazilian Ronaldo and let's hope that he will not be at this level at the European Championships if he plays against Spain."
Questioned about Friday's Champions League draw, the 30-year-old custodian admitted that he is not worried about the prospect of Madrid being drawn with Barcelona in the quarter-finals.
"We were not ready to play four Madrid-Barca games last season and perhaps it had an impact on the Spain national team," Casillas said.
"We were lucky that there was not a World Cup or a Euro last year, but we are friends and all is forgiven when we sit down and have a Coke together.
"Apart from the interests of our respective clubs, we have a country behind us and what better joy than to give this country another Euro trophy?
"Things have been discussed and there is no problem. We will only think about the national team when the time comes, despite the fact that we are now thinking about our clubs.
"When the Euros start, we will have forgotten about all that happened before."
Casillas then came out in defense of under-fire striker Fernando Torres, who was left out of Spain's squad for February's friendly against Venezuela as a result of his poor moment of form at Chelsea.
"People forget quickly about all the important things he has done for the national team," he noted.
"People are unfair with him, he has earned the right to be trusted more."
The Madrid captain went on to reveal that he hopes to be fit to make Spain's squad at Euro 2016 as he is not thinking about hanging up his gloves.
"From my perspective, it is clear that this will not be my last Euro tournament, unless the coach does not count on me," Casillas explained.
"A goalkeeper can play at a top level for more years, but perhaps two more Euros will be complicated."
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