Seeing it and being able to do anything about it are two vastly different things.

Kind of like seeing a train coming at you and you're on the tracks. Seeing the train vs your ability to stop the train "on time" are two different situations.

We'd likely know weeks/months/years in advance. The mass/composition of the asteroid and the lead time we have dramatically impact our ability to do anything about it.

If it's "small" and we have 20+ years heads up, and it's trajectory is favorable, I like to think we could come together for a solution.

If we learn of it month's out, there's nothing we we could really do aside from prep to reboot civilization after impact.