You could say that about any game though. Sunderland and Newcastle have beaten some of the best teams in Europe in this league, but lost to some of the teams facing relegation. It means nothing on the day. Suarez and Carroll could only put one past you last week, but Ameobi could come along on Saturday and put three past you. It's just impossible to tell what will happen and none of these 'rules' or theories really work.
Obertan wasn't anything amazing, but he's better than expected and a much better player than I've seen for Man Utd. Arsenal are hard to beat and have a good defence, so the fact that Obertan couldn't get through them in the first game of the season doesn't bother me a great deal. Might be 5x better on the derby day, might be 5x worse. We'll wait and see, but I'm just glad the potential is there to do some damage.
What's scoring last week got to do with ANYTHING? "We scored in our last match and you didn't, so we'll just shade it" makes absolutely no sense.
Your theories are all well and good, but you WILL get beaten by some mid-table and bottom-table teams this year, so let's see how those theories hold up then.