What condition was the impeller in? The photo looks like the top seal from the water pump housing. If water flow stops the hot exhaust gases flowing over the water pump cause it to melt. In some conditions the pump housing can partially melt before the engine overheats. You obviously need to replace the entire pump assembly.
If the impeller is missing blades you need to find those. They are somewhere downstream in the cooling system and could cause a partial blockage if you don't remove them.