The youngster was offered new terms by Real, but turned them down when United came calling.
There was disappointment inside Real Madrid when Fernandez chose to quit for United, but the decision is already paying off for the youngster.
Fernandez, 17, is now featuring for United's U23 team. The left-back played in Friday's Premier League Two victory over Everton - and his former coaches at Real aren't surprised.
One coaching source told AS: "Álvaro is a footballer with a great future who has been hurt by having Miguel Gutiérrez in front of him.
"But the conditions for him to do well in England, he has them, his pace and his centres will be appreciated there."
Miguel Gutierrez is a left-back like Alvaro and helped Real Madrid win the UEFA Youth League last season. The 19 year-old was training with Zinedine Zidane's senior Real Madrid team towards the end of last season and has continued to do so this term.
- updated October 27