The 24-year-old made the comments after scoring an 89th-minute winner in Real's unconvincing 2-1 win over Las Palmas on Sunday.
"We know we didn't do well, but we will take three points," he told Sport. "There are days when we're not able to be at our best, but also it is not easy to play at Las Palmas—they did very well and other big teams have struggled here, too. People criticise us when we play badly because we're the best team in the world."
Meanwhile, Casemiro hasn't given up on Los Blancos' chances of winning La Liga despite the 12-point gap between Real Madrid and leaders Barcelona.
"Of course [La Liga] isn't lost," he said. "We are going to fight until the end, even if it does look very difficult."