The Spanish shot-stopper has not conceded a goal in his last five outings for United, leading to Herrera claiming he has 'never seen someone like him'.
"He's a genius. Purely on talent, I've never seen someone like him and we are lucky to have him," Herrera told Catalunya Radio.
"The talent he has, very few have it in the world and David is one of them.
"It's a blessing to have him close, and hopefully it will be for a long time."