The Gunners open their Premier League season at Old Trafford with both teams boasting several new additions.
Timber said: “It is a tough beginning, isn’t it? I think last season we had a tough start. It can go both ways. If you start well, it can be amazing. That is the challenge.
“Personally, when the programme comes out for the first time you think, ‘oh, that is a tough start’.
“And then, for a whole week you are just talking about Manchester United. Not the game against City. It is so far away. The whole week has been about Manchester United. Not even Leeds or Liverpool. United have got some good players. They have a great squad and great manager.
“Last season we had a tough game away there and it will be tough again on Sunday — but also because they have new players, you don’t know what to expect. That’s the thing with the Premier League that I find so impressive.
“Every team is getting stronger every year. United were 14th last year and you look at that team and think, ‘how is that possible?’. Everyone is expecting a lot from them.
“Chelsea are looking amazing and there are so many good teams, it is going to be tough for sure. That’s why it is so important to focus on ourselves and believe in what we can do.
"We can’t look at too many teams and what is happening around the league. I don’t think that would be good for us or for anyone.”