Xu Nuo was worn out, because of Master Merlin's mistake. When he was worn out for the second time, it was still the HP world, but Xu Nuo became Salazar Slytherin, and then became the green-eyed savior. Please see how he sets off the butterfly effect and subverts the plot.
(The author is very lazy, so be cautious when jumping into pits. CP Team Hero. Any similarities are purely coincidental)