The young striker had a trial with our city rivals but United lost interest in signing him when work permit problems came up.
"I've still got photos with Sir Alex and all of the players, but it was just a work permit which stopped me playing for them. Maybe now I can get my revenge on them with City," said Castillo.
Red tape also looks set to delay Castillo's City debut. The 23-year-old will be eligible to play at Newcastle on January 2, but with New Year's Day being a Bank Holiday, he won't be registred in time.
Castillo is therefore hoping to make his debut in the FA Cup Third Round clash at West Ham on January 5.
After the United move fell through, Castillo accepted an offer to join Olympiacos, where he spent 7 years and gained Greek citizenship, before moving to Ukraine with Elano's former club Shakhtar Donetsk in the summer.
But Castillo failed to score in 8 games for Shakhtar and was so eager to join the Blues he even paid towards the fee which secured his year-long loan deal.
"I knew from the very first moment that I came to the club that this is where I want to play my football," he told the City website.
"It was a frustrating time at Shakhtar and, yes, I helped make the move happen, but that shows you how much I want to play football and want to play for Manchester City.
"I've been very impressed with everything I've seen here. The manager clearly is a great manager and the club is going in a positive direction and I want to be part of that."