I've been looking at a stat I call usage rate for some time, which measures the percentage of a team's possessions a player "uses", and by use I mean the times they are involved in the final action of the possession (shot, key pass, assist, misplaced pass, or unsuccessful dribble) adjusted for minutes played. The stat helps find which players are most involved in a team's attack, and I've included the numbers for Kelechi and Rashford below:
Last season, Kelechi had a usage rate of 13.2%, whereas Rashford had a usage rate of just 9.18%. This signifies Kelechi had a lot more involvement in his team's play than did Rashford, which is justified by the eye test. Kelechi's eye for a pass and ability to hold onto the ball in tight spaces has been as clear as his finishing ability, but that hasn't really been true for Rashford. By this measure, Rashford profiles as more of a goal-poacher who makes good runs into the box and can finish chances, but is less involved in overall play.
This is not to say that Rashford won''t be a good player. You can certainly have a great career with a low usage rate if you operate with a high level of efficiency. Jermaine Defoe springs to mind as an example, as his usage rate was just 7.62% this season yet scored 15 goals and created 21 chances. However, a low usage rate sets the margin for error very low, and currently Rashford is not particularly efficient. One of the things scouts loved were his direct runs, but in the PL so far that has mostly translated into a lot of turnovers as the numbers show. Another thing I like to look at is the percentage of possessions a player uses that have a positive outcome (i.e. key pass, assist, or shot). Rashford's had just 23.5% with a positive outcome, whereas Kelechi's had 33%, showing Rashford has been less efficient with the possessions he has used. One potential reason that Rashford's per 90 numbers haven't matched Kelechi's is that United's system is such that it deflates overall shot numbers by emphasizing possession too much. However, given his stats also don't look very good on a per possession basis, that critique loses some of its power. He is only 18, a year younger than Kelechi, so there's a definite possibility of improvement with time, and having Mourinho coach him instead of Van Gaal can't hurt. He can still be a great player, it's just at this point the data aren't showing him to be one.
Kelechi on the other hand has already proven he is a player that can be a focal point of the offense and still perform efficiently. His usage rate and the percentage of possessions with a positive outcome both ranked in the top five of Manchester City players, which was true of no one else on the team. When combined with his excellent xG numbers, which indicate his shot locations are as good as his overall shot numbers, all the indicators are that he is on the verge of stardom. Aubameyang rumors continue to swirl around, but the way Kelechi has performed so far in his young career we may not even need him.