Personally, I think King has been the best player so far for the reserves this season.
In 9 starts in all competitions, a few of which were on the left in a 4231, he has 8 assists and 3 goals. No one else on the team has more than 4 assists, and only Macheda, Lingard and Cole have more goals (8, 4 and 4 respectively).
King has started all 5 U21 Premier League games, more than anyone else, and has a goal and 3 assists, leading the team in assists in the competition. Only Macheda has more than 1 goal in those 5 games.