The teenager has joined Rangers on loan for the rest of the season.
The 19-year old joins Rangers on a six-month loan just a year after moving to Old Trafford.
Diallo has made nine appearances for the Red Devils in all competitions and scored one goal.
He was on course to join Feyenoord on a season-long loan but a thigh injury in training stalled that move.
This is a straight loan deal with no option or obligation to sign the winger on a permanent deal.
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Diallo said on Sky Sports: “I am delighted to join Rangers until the end of the season.
“It is fantastic for me at this stage of my career to have the opportunity to come to such a big club that is challenging on a number of fronts and will give me some excellent opportunities to test myself in many environments.
“I look forward to training with my teammates for the first-time tomorrow and to meeting the fans for the first time at Ross County on Saturday.”