Every Muslim has to admit this, but none of them really believe that.
And since Islam was revealed to Mohammed, you do have to think that he's perfect because if he's not, it might be not only that he has flaws but that he lied. Or, at the very least, made mistakes about Allah's will.
Mohammed was a Prophet, not the Son of God; a man, not an angel. This is, anyway, a basic teaching of Islam. Ironically, they idolize him more by refusing to let him be portrayed physically than by allowing this. They treat him as though HE were the angel Gabriel.