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Martial is still a good player and the problem has not been his injury, the problem is Manchester United and their manager. No matter which player goes there, it ends badly. The best examples are Maguire or Antony, both were top players before they came to ManU who got into big trouble there. Even CR7 didn't last long at ManU, so you shouldn't look for the problem at the players. As long as the management and the coach don't change, it doesn't matter which players come, ManU will lack success.Actually this is a sad truth but sometimes this is not really the problem of Manchester United like the aspect of Anthony but in Harry Maguire I still think it's both here, because in the aspect of Anthony we can see he is still in good terms with the manager and also from the fact that they rated him above Sancho in the picking order before his loan move to Chelsea.
While the Cr7 issues were thanks to Manchester United and then the media escalated everything and that season was really a nightmare for CR7 especially in the World Cup.
In the aspect of Martial I think we will be able to see if Manchester United was the main problem if he can do well to get his career back on track.