Club president Dan Hunt has warned the Premier League side off the Colombian winger.
The likes of the The Sun have recently claimed that the 23-year-old Colombian winger is on the Premier League side’s radar and is believed to be the subject of a £4 million bid.
However, asked about Swansea’s alleged interest during a Facebook Q&A session with Dallas fans, Hunt was quick to stress that their star man is not for sale.
“We have Fabian Castillo signed through 2019 and we are not interested in selling key pieces of this franchise,” said Hunt. “We are trying to build a champion for many years to come starting with one this year.
“Fabian is a terrific player and I can promise you this isn't the last rumour we will hear. I hope he gets meaningful minutes tomorrow for the Colombian national team against Uruguay.”
Castillo, 23, originally joined Dallas from boyhood club Deportivo Cali in March 2011 and has since scored 29 goals in 135 MLS regular-season appearances, while adding 24 assists.
After a breakout 2014 campaign, in which he bagged a career-high 10 goals, the emerging speedster has taken another big step forward this term, recording nine goals and eight assists in 28 games to earn his senior international debut last month and a first MLS All-Star Game appearance against Tottenham Hotspur in the summer.
Currently away on World Cup qualifying duty with Colombia, the two-time capped youngster was named the league’s top prospect on their annual “24 Under 24” list last month and European interest in his signature now appears to be hotting up a few months before the winter transfer window opens.
Having signed a new deal until 2019 ahead of the 2015 season, Dallas are in no desperate need to sell but, with Castillo himself said to harbour Premier League ambitions, a big enough offer from someone like Swansea might be hard to turn down.
The fact he has only just broken into the national team may hamper his ability to get a work permit at this point, but a potential wing pairing of him and Jefferson Montero would without doubt prove a real nightmare for even English top-flight defenders.
In the meantime, though, he has a great chance of helping Dallas claim their first-ever MLS Cup title, as the Texas outfit currently sit in the Western Conference’s top play-off spot and are in Supporters’ Shield contention ahead of their final three regular-season games.