I think that this isn't the start of a decline for Arsenal at all. Just because they lost points in one match we shouldn't doubt them quickly.

So far they lost points in only two matches and one of them was a defeat against Manchester United in Old Trafford. I don't blame them too much for that loss also because Manchester United were really great at keeping them at the level they wanted. Of course Arsenal will need to improve their performance in big matches to be able to become more serious for the league title. Winning against smaller teams isn't enough for a team to become the champions in the end. Soon there is a Chelsea - Arsenal match also. We will see how good they will be there.