In order to begin to ponder this, one must first see exactly how it did turn out for the parties involved.
Mikel's time in London was one littered with successes: a Champions League success from two final appearances, one Europa League winners’ medal, two Premier League titles, two League Cups and four FA Cups. His influence at Stamford Bridge also rose steadily, and whereas he fulfilled a more sparing role in the early years, come the improbable run to glory in Europe in 2012 he had become firmly established as a first-teamer.
For their part, Manchester United went on to play in three Champions League finals, winning one (incidentally, against Chelsea); they also amassed five league titles, one FA Cup and two League Cup triumphs.