None of us can say for sure what his issue is, probably not even him. However, I just feel Rashford has met the goals that drove him to the "top" in football and while he probably tries to motivate himself or convince himself that he is motivated, it's just not there.
He came from a very underprivileged childhood and he's talked about how that has driven him in life. I wonder if the fulfillment of the need to sort his family out for life and to give back to the community means he's met his intrinsic goals and his heart's just not in it enough to "suffer" for the team for the sake of winning.
He probably still enjoys football, but not the harsh demands, pressures, and expectations of top level professional football. Rio mentioned being surprised at seeing him in training laughing and messing about with Bruno while they were doing some extra work while the main session had finished, which was in stark contrast to how sullen he looks on game day.
If this is actually the case that his motivation is gone, and I was in his shoes, I'm unsure what I'd do about it considering the fat paycheck guaranteed over the next few years.
This is obviously just an opinion based on observing his game and the limited glimpses we get of his life outside the pitch.