He's probably the unluckiest player currently in world football in that regard. He already publicly considered retirement aged 23. It seems everytime he gets going, he's out for a few months again and has to start all over again. If your training routine consists to 90+% of getting fit again, there's also not much time to work on your weaknesses. That being said, Sancho is a similarly good dribbler, not injury-prone and already has end product like few others. That's why he is so special.
However, I don't really see where Rashford's statistics are in a different league to Coman's. He's currently at 17 goals and 8 assists in roughly 2500 minutes, so more or less 100 minutes per scorer. However, 6 of those goals are penalties which neither Coman nor Sancho take. Those aside his quota goes up to 131 minutes per scorer. Last season exemplarily, Coman was at 127 minutes per scorer, so slightly better.
Trust me, if you watch him regularly, you'll get the hype. He's an incredible talent. If he moves club and gets his injuries under control, this could be a Robben story 2.0. Wouldn't bet on that, though.