Alex Ferguson admitted prior to the Basel defeat that 'complacency' and 'casualness' has cost United in Europe this season.
Without a fellow heavyweight in the group, United approached too many games too lightly, with Ferguson's rotation policy enabling Basel and Benfica to claim points at Old Trafford which ultimately cost United qualification.
2 DEFENSIVE PROBLEMS
It seems a long time since United coasted through their group last season without even conceding a goal. This season, they have shipped eight goals in six games. Edwin van der Sar's retirement and injuries to Nemanja Vidic and Rio Ferdinand (pictured) have contributed to the new uncertainty, but the lack of cover from midfield has not helped either.
Although United should still have emerged from Group C, injuries have hit them hard.
Tom Cleverley's early season form has been checked by ankle problems, while Anderson, Rio Ferdinand, Nemanja Vidic and Javier Hernandez have all been absent for key games.
4 SUMMER INVESTMENT
United's priority during the summer was to secure the attacking midfielder to replace Paul Scholes, but both Samir Nasri (pictured) and Wesley Sneijder failed to arrive at Old Trafford.
The task now will be to find the creative force capable of unlocking tight European defences which United have lacked since Cristiano Ronaldo's £80m sale to Real Madrid in 2009.
5 WAYNE ROONEY
Rooney is undoubtedly United's match-winner and talisman, but at 26, he is not influencing and winning games as his reputation suggests he should.
Basel was the perfect opportunity to drag United into the last 16, but while Roy Keane and Bryan Robson secured iconic status with inspirational displays against the likes of Juventus and Barcelona, Rooney has still to do the same in Europe.