British Airways is offering return flights between Manchester and London Gatwick on Sunday November 8 - the day United play at Chelsea - for £76.50.
Flight time each way is an hour. So with a 16:00 kick off, you can fly at 06:40 or 12:20 on the way there and at 20:30 on the way back, for £39.60 there and £36.90 back.
You can then get the 35-minute train from Gatwick to Victoria (£10 return in advance, £11 on the day) and then get the Tube five stops south west on the District Line to Fulham Broadway for £4 (a two-zone day ticket).
For domestic flights, you only have to check in an hour before the flight and, in many cases, you can check online and go straight through security tothe game. So as long as you arrive by 19:50, you'll be fine.
For more information about flights, visit Skyscanner.