We never know really.
I meant you see Mahrez, Sterling, Grealish who are more technical efficient than Rashford all had found some rough patch under Pep. Then we saw Sane got cashed out. Gabriel Jesus career low stagnate. Good progressive coach ain't charity worker either. There is certain level of expectation for effort.
Even when look at someone less extreme than Pep like Poch, you can still find players like Dele Alli who had meteoric rise then dropping off the cliff and never recover. Davinson Sanchez their expensive CB signing never really meet the expectation, or Trippier had a stinker season after last World Cup and which got sold. Trippier blamed Tottenham for pushing him to play when he needed treatment!
Klopp had a young Immobile from Serie A, but failed to solve the mystery why he is not performing outside of Italian league despite he has all the right attribute. Didn't develop Perisic at Dortmund for how useful his later role turned out to be. Perisic had to learn the hard way failing for years as a poor man Robben, until he buckled up his idea and become a team player, playing a supporting role, and ending up a hard worker from a selfish lazy player he used to be.
What done is done. I don't see how at their age, Martial and Rashford are right in throwing in the towel and self pitying. It would only confirm that they indeed have weak mentality.