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.
The fact that Rashford ended up at Barcelona is a big stroke of luck for him, and how he will be able to take advantage of it will depend on him. There is a lot of competition at Barcelona, and the fact that Flick is giving him playing time shows that he trusts him, or at least wants to see what he is capable of. I think that out of all the possible options, this was the best one for Rashford, and if he somehow manages to become a first-team player, I will be happy for him.