By Robbie Fowler Liverpool and Manchester United are the two most successful clubs in English football. Winning titles and trophies is in their DNA. But they are also the country’s two biggest rivals. And sometimes, even when one club isn’t quite at their best, an opportunity arises for them to stop the old enemy in their tracks. That is what’s at stake at Old Trafford today. United can still win the FA Cup – ironically after landing a sucker punch to knock Liverpool out in the quarter-finals in a 4-3 thriller last month. But Liverpool are around the top of the Premier League and will equal …