The 18-year-old Chile Under-23 international completed his move to the Premier League giants from Universidad de Chile on Wednesday.
"We scouted him when he was 14 years old and just monitored him," Ferguson told the club's official magazine Inside United.
"When he turned 18, we knew we could take him; it's the first time we can sign them in this country, at 18, and so we were very fortunate the Chile Under-20s were over here.
"They were in tournaments at the Milk Cup (in Northern Ireland) and over in Holland. We watched all the games and everyone said: 'You need to bring him in'."
Ferguson also expressed his confidence in the 18-year-old to succeed at Old Trafford.
"He's very quick and a good finisher," he said. "He's strong, with great shoulders on him for a boy of 18. So hopefully he'll do fine."
Henriquez scored 11 goals in 17 matches for Universidad de Chile before joining United.