The former Malaga midfielder has impressed for Real this term, helping fill the void left by Luka Modric's injury-enforced absence to represent a key part of Carlo Ancelotti's title-chasing side.
The 22-year-old was particularly influential in Real's 2-0 win over Elche on Sunday - setting up Cristiano Ronaldo for the second goal - and Casillas has no doubts over the youngster's talent.
"He is the most important player for the future of Spanish football," Casillas told Canal Plus.
"He has to focus on his game because flattery can weaken players."
Real have lost just one game in 10 following their success at Estadio Manuel Martinez Valero, with Ancelotti's side recovering well from their 4-0 derby mauling at the hands of Atletico Madrid.
Casillas - who made his 500th La Liga appearance on Sunday - feels Real are back on track.
"We're getting there. In two weeks football can change a lot. Before there was a barrage of criticism and now things are quiet," he added.
"We know that at Real Madrid the criticism must be harsh and sometimes it can be soft. We have recovered from the puncture at the Calderon and we are where we want to be.
"We have always had attitude. It’s normal to have ups and downs. In the good times things are easy but there must be more unity in the bad times. That’s what we have done."
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