Man Utd's Achilles' Heel

In the ever optimistic world of football, Manchester United cannot seem to do any wrong, just like how Chelsea cannot do anything wrong when they bought Shevchenko and Ballack last season. Nah...I believe in Alex Ferguson but if they are not careful and they put their concentrations on chasing a quad of trophies, they may just end up with none. Why? Football is a funny game, 1+1 isn't always equal to 2. Having new attacking players does not mean you are bound to improve your squad. So after talking so much, what exactly will cause Man Utd's downfall - their defence. They may have brought in some of the best talents around in Hargreaves, Nani and Anderson, but if something does happen to Vidic, Ferdinand, Evra or Gary Neville, it may just jeopardize their season. Remember when Ferdinand was banned for a year and Man Utd's sparkling start came to a disastrous end? It is similar to how Chelsea's lose of John Terry and Petr Cech last season marked of the beginning of their downfall last season. And goodness me, Ferguson has not brought in much defensive cover and Heinze, probably the only other person able to play on the left hand side as a defender is threatening to go away. Which leaves Chris Eagles, Wes Brown, John O'Shea and Silvestre. Seriously, I don't trust them.