The Dutchman has been struggling with anaemia problems recently.
“Mourinho is in the past,” Sneijder wrote on his Twitter account. “We must think about the present and no longer in the past.
“We will get over these moments of fatigue, and I will be at my best soon. This rest has helped me to relax and reflect.
“We might be in some trouble, but there are explanations for this. We come from a fantastic season that will be difficult to forget. The real fans will never jeer their own players, especially when they are going through a tough period.
“I don’t want to lose the derby because it was my debut match last season. I will never forget it. I am getting better and need help from the fans and Benitez. He is a great coach just like Mourinho.”