Not specific to Martial, our injury issues might run deeper. The medical staff, physios, sports scientists at the club all went to essentially the same courses and have same knowledge as those at other clubs.
I think some of this is due to our position in the table. These players have basically nothing to play for, we are not going to win the league or finish top 4, we are not going to get relegated.
It is common knowledge that professional sports people regularly play through minor niggles and pain when there is something on the line. For example at Liverpool now, you will rush back in case you lose your starting place, miss out on points for the team, you miss the thrill of every game being part of a title hunt. The physios would have to hold you back.
But if you are Man United player now, why would you do that? You might be more inclined to take 4 weeks recovery, when really you could be back in 2. Or just sit out a game if you have a sniffle of a cold!
Sure what will you miss out on?! Another underwhelming draw/loss where your reputation takes a hit, the risk recurrence of injury, your value dropping in eyes of other clubs. The incentive to come back from injury or play through pain just isn't there.
This will impact some players more than others, Rashford maybe the opposite and he will carry on through pain. But as a whole motivation within the squad must be low and that just makes it more likely that players sit out more games.