It wouldn't cause major chaos. Disrupting the entire London Underground network for days - now
that's chaos. Closing down a football stadiums for 24 hours - hell, that's just a minor irritation. People
need transport. They don't
need football stadiums. When's the last time you heard of the City being paralysed because Stamford Bridge was closed?
What sort of device did you have in mind here? Something that can kill thousands of people yet is still so small you can smuggle it into a football ground undetected? And what would that be, exactly?

Even the suicide bombers in Iraq can't kill thousands of people at once - and they're using car bombs in the marketplace, for crying out loud!
The casualty rate is necessarily restricted by the available technology. You'd have better luck trying to crash a plane into Old Trafford. That method, at least, offers casualties in the thousands.