Complex sentence: Appointed as Busby's successor in June 1969 at the age of 31, at a time when the Manchester United side was in transition and Busby had moved upstairs to become General Manager, McGuinness's reign was not as successful as was hoped.
Split into simpler sentences: Appointed as Busby's successor in June 1969 at the age of 31, he endured a largely dismal reign. At a time when the Manchester United side was in transition and Busby had moved to General Manager, McGuinness's reign as Manchester United manager was not as successful as was hoped.