The Gunners meet United in the Third Round after defeat at home in the first-leg of their Carabao Cup semifinal with Newcastle.
Timber said: “It was already a big game because United always is. We want to win and give the fans something to celebrate so we are going to give our all on Sunday. I am really looking forward to that game and to bounce back. It is another cup game so we have to win.
“Every competition is really important. We want to win everything. It is not easy, and it is not always going to happen, but that is why it is important that we bounce back. The focus is on the FA Cup and then, after that, the Premier League.”
Timber added: “We played them already a month ago so we know what we are getting. They have a good squad with a lot of good players. It is not going to be easy but we will be ready.”