Nani has also moved to Sporting on loan for the rest of the season, rejoining the club from which he moved to Old Trafford in the summer of 2007, although his parent club will continue to pay his wages.
It will be the responsibility of Rojo to ensure that his new side do not concede any cheap goals. He played in the full 120 minutes of Argentina's defeat by Germany in the World Cup final, occupying the left-back position for his country during the competition. He is similarly comfortable at centre-back, though, and could also be given the chance to help plug the gap following the departures of Rio Ferdinand and Nemanja Vidic.
Now 24, Rojo began his career in his homeland with Estudiantes de la Plata and spent a single season with Spartak Moscow before moving to Sporting in 2012. "It is such an honour to say that I now play for Manchester United," he said last night. "The Premier League is the most exciting league in the world and to have the chance to play in it for the world's biggest club is a dream for me.
"I am young and am very keen to continue to learn the game, so playing for such an experienced coach as Louis van Gaal is a fantastic opportunity for me. I have joined United to work hard to win trophies and I know that the manager shares that ambition."
One of his new team-mates has a very different aspiration this season, though. Marouane Fellaini is understood to have suffered an injury in training and was seen last night on a pair of crutches. The Belgian - who has been an unpopular player among fans, being booed during a pre-season match with Valencia - was closing on a loan move to Napoli until the end of the season but that transfer is now in serious doubt as the midfielder awaits news on the extend of his injury.
His injury would also appear to hurt United somewhat, a club which has only Ander Herrera, Tom Cleverley and Darren Fletcher to play in central midfield. Michael Carrick will be sidelined for another two months as a consequence of an ankle injury.
He is not the only midfielder in the English top flight to be treading lightly, either, since Jack Wilshere has reminded his Arsenal side not to lose focus following a goalless draw in the first leg of their Champions League play-off against Besiktas.
The London club will host the second leg next week and are expected to overcome their Turkish opponents and reach the group stages of the competition, yet Wilshere remains wary. "It was a hostile atmosphere, but I think we battled well and showed a different side to our game, especially at the end when we went down to 10 men, it was tough, so we did well," said the Arsenal midfielder, whose side lost Aaron Ramsey to a red card.
"They worked just as hard as we did last year to get themselves into this position, so they are not going to just turn up [at the Emirates] and throw all of that hard work away. It is not going to be an easy game and we can't take it for granted.
"They are going to come full of confidence from their performance in the first game and we are going to have to start how they did and take the game to them."
As Arsenal continue to look to Europe, the attention of city rivals Queens Park Rangers was held much closer to home yesterday after the signing of Leroy Fer from Norwich City. The 24-year-old has signed a three-year contract at Loftus Road.
"Leroy's arrival is a massive boost for us," said Harry Redknapp, the QPR manager. "He's another excellent signing and I'm delighted we've been able to get him to QPR."