Article updated 11 June
Here, we assess the highly-rated youngster and learn more about his promising career...
The basics: Varela, who was born in Montevideo on 24 March 1993, is an ambitious full-back who plays on the right flank for South American giants Penarol in his native Uruguay.
The background: The 20-year-old has made one first-team appearance at the Centenario Stadium but is a regular for Uruguay's Under-20s with nine caps to his name.
The links: Guillermo recently completed a trial at Carrington and later spoke positively about the experience to South American radio station Tirando Paredes: “I have trained with the first-team. I am very happy and I was received very well. It was a surprise to get the chance to test myself with Manchester United. I'm glad and happy, and grateful to Penarol who allowed me to travel. I was treated so well and Chicharito helped me a lot.”
The manager: “This is what he deserves. It is a shame to see him go but you can’t deny him the opportunity to join a club like this.” – Penarol head coach Jorge Da Silva.
The future: After reaching an agreement at Carrington on Friday, Varela's next step is to fly to Turkey to represent Uruguay at the World Under-20 Championships – where future team-mates Angelo Henriquez (Chile), Sam Johnstone, Tom Thorpe and Larnell Cole (England) are also taking part. The