The 23-year-old Colombia international is reportedly on Garry Monk's radar.
Arguably the fastest player in MLS, Castillo originally joined Dallas from boyhood club Deportivo Cali in March 2011 and has since scored 28 goals in 135 regular-season appearances for the Texas-based outfit, while adding 24 assists in the process.
The 23-year-old enjoyed a breakout 2014 campaign, bagging a career-high 10 goals, and he has arguably been even more impressive 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.
Not only has his stellar form helped Dallas move into the Western Conference’s top play-off spot and Supporters’ Shield contention with just three regular-season matches left but the likes of The Sun have reported that Swansea are now in hot pursuit of his signature.
Garry Monk’s side are rumoured to have already lodged a £4 million bid for the pacey wide man a few months before the winter transfer window opens and, given the considerable promise he has shown in the States, that price would be a real bargain if it proves enough to lure him across the Atlantic.
Capable of terrorising defenders with his abundance of speed and skill, Castillo came to MLS as a raw 18-year-old with physical gifts and little more, but he has steadily worked on his decision-making to blossom into one of the league’s most feared offensive threats – one who can change a game in an instant.
Comparable to Swansea’s own South American wide man Jefferson Montero, he is at his best when using his trickery to go at players one-on-one and that certainly shows up in his statistics this season.
In his 28 games, the youngster has attempted a league-leading 202 dribbles – 53 more than any other player, while his once-questionable finishing has also come on in leaps and bounds over the past two campaigns.
Colombia have traditionally shied away from calling up MLS-based players to the national team but it is testament to the two-time capped attacker’s talent that he had now broken into a squad featuring the likes Radamel Falcao, Carlos Bacca, Jackson Martinez and Teofilo Gutierrez, and he should be joining them all in Europe before long.
Rated the top prospect in MLS on the league’s annual “24 Under 24” list last month, Dallas penned their star man to a deal until 2019 ahead of this season but, with The Sun’s report claiming that he harbours Premier League ambitions, an enticing enough bid from someone like Swansea might be hard to turn down.
He undoubtedly has the ability to succeed at the Liberty Stadium under Monk’s guidance and the thought of pairing him and Ecuador ace Montero on the flanks is a tantalising prospect.
He may not be quite the finished article yet, having struggled with consistency at times during 2015, but if a deal and work permit can be wrapped up for Castillo then the Swans would have quite a talent on their hands.