I'd rather get some more firepower than spend money on Hummels. Look at Milan, they actually don't have a great defense. But they're still winning titles. They have Moses (Nesta) at the back, with people like Abate (is this even a football player?) and grandpa Zambrotta. And Bonera, who's just terrible. The only really good defender they have is Thiago Silva.
Don't get me wrong, I have nothing against having a great defense, but it's about prioritizing.
What we lack, and what's gonna be the reason we'll come up short this season, is that we're not scoring enough goals.