I don't think he'll show any sign of laziness cause if you want to be a serious player and get playing time at Barcelona then you'll have to be very hardworking on and off the pitch especially on the pitch, even Lewandowski that's older workers hard then why wouldn't Rashford, Hansi Flick is a no nonsense coach be won't tolerate any form of laziness from Rashford, I believe that on his first training he'll see for himself and see reasons to give his best so they'll be permanently buy him into the team, failure to do so would get him sent back to Manchester United and Rashford won't want that.
Because Lewandowski was hardworking even before he came to Barcelona. Look at how he worked at Bayern, he was in great shape and scored a lot of goals. At Barcelona, he tries to do the same, but obviously it is harder for him because he is not getting younger and he has to compete with younger players. But I recently saw how Lewandowski trains and I can’t say he is lacking in anything, although it is clear that it will get harder for him now. By the way, Rashford was lazy at Manchester United, and I don’t see him trying much harder at Aston Villa, so it also depends on the player.