Arsene Wenger believes one or two key decisions have impacted on United's recent travails.
United finished seventh in the Premier League under David Moyes' guidance last term and neither made the Champions League nor Europa League as a result.
And Wenger believes 'one or two average decisions' led to the downfall, proving just how fragile even the biggest clubs can be in the modern day.
Asked whether the Gunners' unbeaten league run 10 years ago was perfectly timed ahead of Chelsea's period of domestic dominance, Wenger said in an extract from a new book about him - published in the Guardian:
"It was just in time. Everybody speaks of the Chelsea team that went on to win the Championship afterwards but in 2004 they already had a super team.
"[Roman] Abramovich came in and injected more money, they brought Didier Drogba and Ricardo Carvalho into a team who already finished second. They took over after that. We had Chelsea coming in, Man City later.
"Look what happened next – now you have Man United not even in the Europa League. You make one or two average decisions and you are out of it."