After the 4-1 defeat against Man City, the only match where ManU can salvage something from the season is the FA cup now. Arsenal having 2 matches against ManU and 2 more vs. Chelsea in 2 weeks over that period, may well place the lowest priority on the FA cup match. This could prove crucial in the outcome.
The disadvantage of being the dominant force of any country is that if you don't win in any season, you are said to be in deep trouble and the dominance is fading and the manager should go.
Look at Real Madrid, if they don't win the Spanish league and the European Cup every season, then they are said to have failed.
The truth is that Manchester United is still the best the football club in England. Being the best does not mean winning every season. It simply means that the club still has the courage and determination to bounce back from a disappointing season (against very high expectations) and go on a winning run again.
Arsenal, and more recently Chelsea, have caught up a lot on Manchester United. However they are still considered to be in the "challengers" status. It's quite obvious because when they fail to win the league, there is far less fuss stirred in the media.
In any case, Arsenal still has a lot to do to win any of the remaining 3 trophies.