But Rangers sporting director Ross Wilson described it as "a really exciting signing for us".

"Amad and United had an array of options this window across the Premier League and in Europe," he said.

Rangers' need for attacking options intensified after Ianis Hagi was ruled out for the rest of the season last week.

"We have been working hard with Manchester United and Amad's agents throughout January to reach an agreement," Wilson added. "We have stayed patient in our discussions as we know what Amad can bring to our group."

It is understood the deal is a straight loan with no option to buy.

Diallo, who became the first player to be signed and registered under post-Brexit rules, was due to join Feyenoord at the start of the season, but the move fell through because of injury.

He has since returned to fitness, but his start in a 1-1 Champions League draw with Young Boys in December was his only game this campaign and United feel he will develop under Rangers manager Giovanni van Bronckhorst.

The Dutchman told his club website: "He is a player who I have been aware of for some time and, when the opportunity to sign him came up, we were very keen to make it happen.

"He will add a lot to our squad and provide even more competition for the players in the forward areas."

Diallo, who had only played four times for Atalanta before his move to the English Premier League, said: "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."

Meanwhile, Dutch midfielder Juninho Bacuna has been sold by Rangers to Birmingham City for an undisclosed fee.

The 24-year-old Curacao international joined the Ibrox club from Huddersfield Town but has made only 12 appearances, seven of them starts, scoring once.