Right. A tournament that history has shown requires a lot of experience. So it's Reed's first Masters, and it's also his major of any kind ... it would not be a stretch to predict that he'll not do well. It would, however, be a stretch for anybody to then say "see, I told you that he would play bad because his arrogance hurt him." That would be silly, because, like you said ... that kind of stuff is not going to bother people who don't worry about what other people think of them, and Reed has yet to show, to this point, that he does worry about what others think of him.
Also, just because he said he's number 5 doesn't add pressure. What ever his mind thinks is what pressure he is going to feel. Heck he could think he's number 5 and not say it and still feel the same pressure even if have or had not said it. Saying something doesn't add anything to it.
I wouldn't go this far. Saying something could certainly add pressure.
Lebron James, for example, could have kept his mouth shut when he went to Miami, and could have retired after last season as a god if he wanted to. But since he promised "not 2, not 3, not 4, not 5, etc, etc" championships, he put more pressure on himself to deliver those. He could think to himself that he's going to win every title and every MVP until he retires, but there is no pressure on him to do that, because nobody is expecting it. Once he tells the world though, there could be some pressure.
There isn't necessarily going to be pressure, because everybody performs differently under different conditions, but there definitely COULD be.