Of course it matters who it's against!
Against a team like Southampton, I agree with you that Nani is going to contribute more toward victory than Park could, because teams like that can't defend him and he'll be very productive. Against better teams however, he is not nearly as "productive", his defense is not that great, his decision making at times is awful, he gives the ball away far too much and his concentration isn't what it should be. Whereas Park, for all the reasons that have been named in this thread, will more often than not play pretty damned well, and contributes more toward the end result than Nani would. Look, I'm not slating Nani, and I hope he'll come good, but you can't possibly take this game as evidence he should move ahead of Park in the rotation. He was excellent, and he has been consistently excellent against weaker sides. but against stronger sides, he has been consistently poor - which means, incidentally, that posters slating him for being inconsistent are off the mark.
So we'll see him midweek against Derby, who are in the bottom half of the Championship, and he'll probably chew them up. Then we'll see Park on Sunday against Chelsea, and there's a pretty good chance he'll do an excellent job against one of the strongest teams in the world. A job similar to the one he did in September, where his
all-around performance was excellent and he scored our only goal in a 1-1 draw.