My breakdown of his United career:
04/05 - 05/6: Young and swashbuckling. A force of nature at times, if a little immature and inconsistent, which was to be expected.
06/07 - 08/9: Often inconsistent in terms of his goal-scoring (he'd score in bursts before going on long runs without a goal), he functioned primarily in a supporting role to allow Ronaldo the freedom to become the best player in the world. Very valuable in this period, yet there was always that nagging feeling that he wasn't being allowed to fulfill his potential.
09/10: We got a taste of what his full potential was this season, and it was about as good as anyone else in football that year, barring the obvious two in Spain.
10/11 - 11/12: This spell began with by far his worst run of form up to this point following his injury at Bayern, a disastrous World Cup, and his transfer request. Played himself into incredible form during the 2011 run-in. The following season he started where he left off, and maintained his goal-scoring form throughout, but in the second half of 11/12 I think we begin to see the first signs of his decline - and I think Fergie also saw them. His general play began to get really sloppy, and his energy levels weren't quite the same.
12/13 - 14/15: For me this is just one long, slow stretch of decline, during which he was still a very useful forward at times, but nowhere near his former self. By the end of 14/15 it was doubtful if he was worth his place in the team at all.
15/16 - 16/17: A liability on the pitch 90% of the time.