He also fired the Red Devils to FA Cup glory, scoring a late winner against Everton in the semi-finals.
However, upon taking the United job, Mourinho made him switch to No 11 instead.
And the MEN say that Martial has been unhappy at the club ever since.
Mourinho instead gave the number to Zlatan Ibrahimovic.
And it has since changed hands again, with Romelu Lukaku the current occupant.
GETTY
Man Utd news: Anthony Martial saw his No 9 shirt given to Zlatan Ibrahimovic
Anthony Martial shows off stunning skills for France
Invalid email
We use your sign-up to provide content in ways you've consented to and to improve our understanding of you. This may include adverts from us and 3rd parties based on our understanding. You can unsubscribe at any time. Read our Privacy Policy
Speaking immediately after the decision, Martial’s agent Philippe Lamboley ripped into the club for the decision to change his number.
“I am not going to lie to you. Yes, this has affected him, yes this surprised him, yes it disappointed him... for a few days,” he said.
“But he is a professional. He then got back into the swing of things.
“Personally, I have not accepted this decision that I deemed uncalled for and disrespectful towards my player.
“This feeling, I have directed this towards Manchester United's directors. We turn the page and move on but it's hard to digest it.”