It's not about being lazy. Rashford really wants to stay with the Barcelona team. He did a lot of fitness, like going to the gym. But he who has lost how good he used to play has lost it. It will be hard for him to get all that back. That's what Rashford is passing through. It's hard for him to accept that as reality that befalls him.
I think Rashford needs to take a break from football to rediscover himself rather than thinking that he is among the best players a team can have. That's what is deceiving him not to accept the reality that he is no longer in form.
I say lazy because he is the type of player who is reluctant to chase the ball when he loses possession. Fitness-wise, that can probably be improved with physical training, but if that lazy trait is one that might be quite difficult to break, because that's just the way Rashford plays. That can only change if Rashford truly wants to change on his own. Otherwise, I don't think he would have been able to succeed at Barcelona.
So it depends on himself and how much he is willing to sacrifice and put in the effort. If he continues to act like he did when he was with Man United, then it will not end well.