What makes you say this?
I do think if Aguero hadn't scored against QPR we'd have flogged Balotelli that summer and probably sacked Mancini, fought harder for RVP and De Rossi and possibly even brought Fernandinho a year earlier. I think we would have won the title and the FA Cup in 2013. Lots of people forget that Fergie was going to retire until Aguero scored, so that would have left the 2013 title race wide open while Chelsea fannied about with Benitez in charge. And if Moyes had gone to United a year earlier as a consequence, Everton probably never would have been knocked out by Wigan in the FA Cup quarter-final that year, leaving us potentially with a final against Everton or Millwall. Then again, we'd have replaced Mancini with Ancelotti (who wasn't exactly a happy bunny at PSG and went to Madrid the next summer) - league titles weren't exactly a speciality for Carlo, so maybe Chelsea would have cobbled something together.