We've seen this soo many times from Rashford, it's not fitness, it's attitude.
He has attitude issues, he has a good couple of months and he suddenly thinks he doesn't have to do the dirty part of the game. He tried to take on his opponent once yesterday, he failed and did not even attempt a dribble after this, he was slow with the ball, just thought he could stand on it and pick out a pass, which he couldn't. Defensively he just doesn't care, the worst pressing footballer in the league and then he throws his hands in the air when it's his lack of pressing that allows the opposition play through.
He honestly needs to humble himself a little.