I think for me there are moments that stick out more in past seasons in regards to Rashford in tough angles not making a simple pass for Martial, and going for the goal, in games where the game was still undecided, but I don't think Rashford is a selfish player, I just think he makes poor decisions at times in the last third. He's assisted Martial plenty of times, and vice versa.
I just don't see the point of having an argument on who is more selfish when they've both helped each other out tremendously last season in reaching their best output and whatever level of selfishness you see in Rashford, it's not egregious enough to be an actual issue for the team or Martial, who could be a bit more selfish himself as a CF.