Herrera had spent the past three seasons at his hometown club Bilbao and he has admitted that if United hadn't shown an interest in him then he may have ended his career there.
"I would never have left Athletic if it hadn't been for a club like United," he told The Guardian.
"I left a different, unique club, with a special philosophy and incredible people. It's a club that few of us have had the privilege of playing for and, who knows, but for United I might have been there 10 years.
"But this is the biggest club in England, a new project, new players and a new coach. Desire too: it's been 25 years and the players are conscious of that. They're determined to put United back where it belongs."
Herrera could make his debut for United during the club's Premier League opener against Swansea City on Saturday.