All the talk of how Martial should fight for his spot is nonsense IMO, I think he's done that this season. The signing of Sanchez, I was very much inclined to believe it would have negative repercussion on Martial. To come back from his terrible second season, coming off the bench and scoring/assisting, claiming he wanted to be a starter but impacting off the bench until Rashford started to lose form and taking the spot from him. Scoring in important games, he's earned this team some points. As far as his inconsistencies or games where he's had little impact, it was very rare that he was the lone culprit in that regard and was more of a reflection of the team as a whole.
Now, fair enough, Sanchez is a much better player than Martial, who could play both on the left or the right but has made his name in the Premier League where Martial was playing. So instead of trying to fit two players on the same team, Mourinho went with Sanchez at the detriment of both Martial and Rashford. Now those are the possible repercussions in losing Martial. It was probably the first thought I had about the Sanchez transfer, that it would relegate Martial to the bench and many thought that was ridiculous and that clearly Mou would shift Sanchez to the right as this was a position of need for the team. Hopefully for the club, Sanchez continues to grow at the club and has at least a couple of seasons playing at his level but his form will dip and we will see where Martial is at that point. I don't think it's obvious he will be either a success or failure, but so far his stats suggest he is an incredibly good player which the eye test doesn't always reflect.