this wasn't the prime english league, this was 2012. on the precipice of a 6-year decline. not just did he do well domestically, he was part of a squad that reached the CL final. his national team record speaks for itself. he made a position for himself in a midfield that had pirlo, vidal, and marchisio. all 3 of them better than any midfielder he would have to compete with at united.
it is also not as though 3-man midfields were unknown to united, whether to accomodate scholes and veron, or in 2007 onwards against bigger teams. neither are rafael and park suited, in any way, shape, or form, to any of the positions in a 2-man midfield. park's deficiencies at that position were exposed in the most brutal fashion possible 6 months before the blackburn match. it's not like the short-term options- cleverley, phil jones, anderson, thrived in that or any other setup. finally, this whole operation cost the club about 90 million pounds and 4 years of desparately hunting for midfielders, we tried and faied with schneiderlin and fellaini too.
the point is that the talent was there all along. not nurturing the talent cost the club in many different ways. (considering the margin at the end of 2011/12, and the result of the blackburn game...)