Man Utd 'complete' fifth deadline day signing as Ed Woodward rounds off frantic day (Image: GETTY)
The teenager has attracted interest from Manchester City and Real Madrid this summer, and held a medical at Carrington earlier on deadline day ahead of a potential move, which has now been confirmed.
Earlier in the day, United completed the signing of teenage defender Willy Kambwala from Ligue 2 side Sochaux.
The 16-year-old is set to join up with the club's under-18 squad.
Pellistri, on the other hand, is set to immediately join up with the senior squad.
Speaking to the club's official website after the deal was confirmed, the teenager admitted that United's policy of giving youth a chance was a key factor in his decision.
He said: “To join a club with the history of Manchester United is a dream come true.
"I have learnt a lot at Penarol and I would like to thank them for everything they have done for me.
"To have the confidence of the manager is an amazing feeling and I know that working with him will be perfect for my development.
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“I am coming into a squad full of great players who I will learn from every day.
"Everyone knows that Manchester United is a club that gives young players an opportunity and I am ready to work hard every day to earn those chances and prove myself in England.”
Ole Gunnar Solskjaer has made no secret of his desire to sign attacking reinforcements this summer, despite observers feeling they have bigger problems in their defence.
And the boss heaped praise on his new youngster, and revealed that the deal was a long time in the making.
Solskjaer said: “Our scouts have followed Facundo for a long time and we believe that he will be a real talent for the long term.
"Whilst we understand it will take time for him to adapt to the English game, he is capable of having an impact with his pace and ability to beat his opponent.
“Facundo has the perfect platform at Manchester United to develop and he has all of the technical qualities and the right mentality required to become a top player here.
"We are all looking forward to working with him to fulfil his potential over the coming years.”
Jadon Sancho had originally been United's top target for the right-wing position, but the club refused to meet Borussia Dortmund's asking price of £108m.
Instead, the club opted to look for younger options with the idea of using them instead of Juan Mata and Jesse Lingard, who he has been forced to use as backup for the usual wide pairing of Mason Greenwood and Marcus Rashford.
Solskjaer also wants to move Greenwood into a more central role, reports say. The teenager has played frequently on the right-wing, but the club's coaching team wants him to play as a centre-forward once he is ready.
Pellistri might not be the last winger through the doors this summer, either.
After failing in their bid to sign Sancho, United turned their attentions towards Barcelona's Ousmane Dembele as an alternative.
The French winger was allowed to leave due to the club's bid to sign Memphis Depay, but ultimately that deal fell through.
Barca wanted to offload Dembele permanently, while United wanted to loan him for a year before making another run at Sancho in 2021.
Now, they are reportedly ready to reignite their interest in Watford's Ismaila Sarr.
The transfer window for deals between Premier League and EFL clubs ends on October 15th, meaning United have time to get the deal done.
They reportedly made a fresh enquiry with Watford on Monday, and are expected to launch another bid.
United were told earlier in the window that it would cost up to £50m to sign the 22-year-old, who was impressive for the Hornets last season.
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