Hire good psychologists and motivators
So that the players would have more desire to win every game or have attitude to win every game and does not get complacent easily. And to get the most of every player or to make each player to play as the best as they can. To improve team morale, player confidence and teamwork too.
Less of dribbling and passing in the vicinity of opposition's box
Sometimes you just need to shoot. When you have the ball in the vicinity of opposition's box, you should just shoot it hard and try to make in on target, especially when wet or slippery conditions. The keeper may save it or may miss it or may save but drop it and the striker could score from the dropped ball.
Rotate players to take corners and freekicks
Sometimes in a match, the same player took all 3 freekicks and and all the freekicks were taken badly, not even on target. In a game usually the team has 2 to 3 players who can take freekicks or corners. So let these players rotate like first player take first 2 freekicks and for the next 2 freekicks other players could take. Don't let only one player take all the freekicks in one game unless the player is really good in taking freekicks.
Buy some good players in January transfer window especially for centre back
As for yesterday game against Man Utd, Arsenal does not have enough CB until need to use LB (Monreal) as CB.
These are players Arsenal could buy in the January transfer window that may strengthen the team:
1 Goalkeepers: Victor Valdes or Ter Stegen or Asmir Begovic
2 Centre Backs: Matt Hummels, Mustafi or Holger Badstuber
2 Midfielders: Griezmann, Khedira
1 Striker: Xherdan Shaqiri
Rotate between these 2 teams. When in a week has 2 games, play first team against the stronger opposition and play second team for the other game.
