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Rashford played under Mourinho for two-and-a-half seasons, scoring 28 goals and winning the League Cup and Europa League.
Under current boss Ole Gunnar Solskjaer, the England international has hit 42 goals for his boyhood club - including 20 in all competitions this season.
During a conversation with former teammate Wayne Rooney for BT Sport's Behind The Lines feature, Rio Ferdinand asked Rashford if there was much freedom for players to interchange positions in the modern game.
That led to Rashford's honest admission regarding his time with Mourinho, explaining how he Portuguese manager was fixed and rigid in his approach.
"I think that's when we play our best football (when we can be flexible)," 23-year-old said.
"Under Jose (Mourinho) I would say everything was a bit (points) "you play there, you play there, you play there". And yeah, you can do a job, but it was difficult for me to play my best football."
The fascinating segment saw Rooney and Rashford talk about their journey and analyse key moments in their career, with direction from Ferdinand.
"Great to play with him as a young lad, and to see the progression he's made so far!"- Football on BT Sport (@btsportfootball) May 9, 2021
Something special dropping for @ManUtd fans tomorrow :red_circle::black_circle:@WayneRooney and @MarcusRashford are the latest guests on @rioferdy5's #BetweenTheLines pic.twitter.com/igMdMsnADn
Interestingly, Rooney revealed that he used Rashford's story to motivate some of the youngsters at Derby.
An 18-year-old Rashford made the most of an injury crisis to seize his opportunity in the United first-team when Louis van Gaal gave him a chance in February 2016.
The Wythenshawe lad and academy product scored twice on his debut against FC Midtjylland in the Europa League, before notching another brace in a dream Premier League debut against Arsenal just days later.
Rooney said: "You can see he [Rashford] has kicked on, it was great to play with him as a young lad coming through.
"You can see the progression he's made so far, it's great to see that.
"Marcus' story from getting in the first team, I used it a couple of weeks ago because we've had injuries at Derby.
"We put a few young lads on the bench who hadn't even trained with us and I used Marcus' story.
"I said Marcus had come in from nowhere, hadn't trained with us, came in and made an impact.
"And look what he's gone on to from that!
"So for those players to try and follow in those footsteps, when you get a chance it's up to you to take it, and he's certainly done that."
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