Personally I see it as poor form brought on through injury, rather than poor form through Nani sizzling it out, which I think is why people get a bit defensive. It was a lapse in form brought about from an injury, that he didn't recover from in time, as oppose to him just playing badly in games. There is a difference, whether that's the case or not I can't say, but I think it's the reason people object to the statement above.
I don't think it's that odd in pointing out the distinction, if you believe it to be the case, given one suggests he couldn't do much, or anything about it, the other says he lacked the form at a crucial stage, which could be down to any number of reasons, fatigue, mentality etc, but it's more in his hands.