If the choice is painkillers and continue to earn millions or no painkillers then i'd guess 99% of young people would take the painkillers.I think it's widely accepted that Rashford suffered a double stress fracture on his back which could be confirmed by him being seen carrying a fracture healing machine. To be honest until this day I still find it weird seeing the photo. Was it a cry for help letting the world know of his injury because surely the responsibility would lie with the medical staff in taking care of any medical equipment and not a player carrying it off the bus? I'm not sharing a conspiracy theory. It's simply just a personal observation that stuck with me which is why for me it's not misremembered the extent of his injury.
On the other hand Rashford himself said it was the first time in a long time he was playing injury free. Of course those type of injuries can still cause discomfort but if that was Rashfords problem last season then no amount of coaching will change the fact he's a spent force unless he's doped up to the eyeballs which would probably threaten his health in the long term. Is it worth it to him in the long run?
Either way I think he has to step back and evaluate what the future holds for him. The club as well before they even think about extending his contract. Lack of motivation or longterm injury worries shold be put under the microscope and acted upon no matter the player involved. If not, nothing is going to change at the club and we'll still be playing catchup another decade down the line.
I haven't seen the picture you are eluding to, but getting used to your equipment in general is fairly normal if you are able to handle it. Since it looked weird i'll assume it is fairly big equipment that someone should have carried for him.