The Spain international lost his place as Madrid's number one to Diego Lopez under Jose Mourinho last season and has thus far been unable to displace the former Villarreal stopper under new coach Carlo Ancelotti.
Buffon is surprised to see Casillas occupying a place among the substitutes, but believes his reaction to losing his spot in the team is great testament to his character and is confident the 32-year-old will bounce back.
"I really admire Iker for who he is. He is still a strong goalkeeper and among the best out there. It is very hard to accept certain situations, but he does just that and I respect that," Buffon was quoted as saying byAS.
"Iker has always been a great champion and great champions always find a way out of negative situations like this. When something like this happens, you draw strength from it.
"I do not know what happened to him last season. I only saw the line-ups. But I think that last term was not just a sporting decision. It had to do with his relationship [with Jose Mourinho].
"I don't know what the problem is now, but I can say that it's very unusual to see a goalkeeper like Iker on the bench. Iker has only become a better man to me for his reaction."
Casillas has made just two official appearances for Madrid so far this term but could feature in the Blancos'clash with Juve in the Champions League on Wednesday.