What are the main problems Manchester United need to fix?

Manchester United have won just one of their first four league games and were dumped out of the League Cup in embarrassing fashion at home to Brighton on Wednesday. Alarm bells are already ringing loudly at Old Trafford given a sluggish, error-strewn start to the campaign. It's only mid-September, and he's only in his first few months as United's manager on a permanent basis, but Michael Carrick is already under serious pressure ahead of their trip to Fulham on Sunday. What are the main problems