Manchester United would have to pay £40m to release Benfica midfielder Nicolas Gaitan from his contract at the Lisbon club.

Sir Alex Ferguson has reportedly been keeping tabs on the 23-year-old who signed a new contract at the Estadio da Luz earlier this week.

Despite tying down Gaitan for five years, the Portuguese club were unable to increase the Argentian's release clause.

The playmaker began his career at Boca Juniors before joining Benfica for £8m in 2010.

He has made six appearances for Argentina at international level.