Are you sure he scored a goal? I remember he took a shot with his left last season, but the one I am remembering didn't score and I don't recall any other.
And that last line is a massive under-statement. Like Mainoldo said, he does everything possible to not use his left. There are plenty of times the left has been the natural foot to use, and he'll change his entire run-up to the ball to use the outside of his right foot or turn his entire body to hit it with his right while off balance.
There are plenty of very one-footed players around. But I can't think of a single one to the extent of Valencia. But it works, so it's not a big deal.