He seems to have developed physically, but what's sticks out is his technical ability on the ball. The first time I saw him play was in a game against Liverpool where we were comfortably beaten and many on here were very critical of our performance and critical of our players. And in that game, Liverpool were physically dominating our players including Mainoo who was physically overpowered on occasions by Liverpool's James Balagizi. But it was Mainoo who I thought was the stand out player in the game due to his touch and technique. The link to the thread below.Looks good, but what is most noticeable is that he looks to be in the midst of much younger players. I.e. he's physically ahead of the curve.
https://www.redcafe.net/threads/u18s-man-utd-v-liverpool.464908/