As has been linked, FIFA's interpretation document sets out how Law 12 is to be interpreted.
Studs up, into the chest of an opponent. Careless? Certainly. Reckless? Maybe. Using excessive force? Well, going in with the studs of your boots aimed directly at the chest of an opponent doesn't help your cause.
I've seen players been given red cards for less than this in the past.
Roy Keane agrees with you:
Heated studio discussion: ITV pundit Roy Keane says Nani deserved his red card against Real Madrid | UEFA Champions League - ITV Sport
"It doesn't matter if he knows there's somebody there or not, it's irrelevant. I don't think the referee actually made the decision because he waited a couple of minutes while the player was getting treatment.
I think it might have been the assistant that made the decision, and I actually think he's made the right call.
We always say referees are very quick to make decisions - he waited a couple of minutes."
- Roy Keane, on ITV.
I personally think that the referee responded correctly. The only people who are kicking up a stink are Man United fans which is understandable because no-one likes to be on the end of a decision like this... even if the referee is right.