It was not a good idea to pick a fight with a murderous Death Eater in his condition... that thought crossed Moody's thoughts. But he justified his actions because that bastard had sneaked up behind him - Rosier would've attacked anyway. The Auror was sluggish, both in body and in mind, so when the cutting curse came, his reflexes were slow. He raised his wand to block the spell, but he couldn't do it in time. The curse struck him across the chest, and the only reason why it didn't dig deeper was that Moody pivoted to physically evade the path of the magic. A red line blossomed upon his clothes, and the blood was washed by the torrents of rain.
The pain helped Moody to focus, so he was able to block the severing curse and counter with a curse, directed at Rosier's eyes, attempting to blind the man.
He realized he was in trouble, and if he wanted to return to settle the score another day, he would have to get out of there.