The 23-year-old Colombian has recently broken into the senior national team.
Castillo, 23, originally joined Dallas in March 2011 from boyhood club Deportivo Cali and has since made 138 MLS appearances, while recording 29 goals and 24 assists in the process.
Arguably the league’s fastest player, the Colombian enjoyed a breakout season in 2014, when he bagged a career-high 10 goals, and he has been even more impressive this term.
As well as scoring nine times and adding eight assists in 28 games, the promising wide man has attempted a hugely impressive 202 dribbles – 53 more than any other player, while he was also voted the league’s No. 1 prospect on their annual “24 Under 24” list last month after making his first All-Star Game appearance in the summer against Tottenham Hotspur.
In addition, while Colombia have traditionally ignored MLS-based players, the skilful speedster’s recent impressive run of form earned him his senior international debut last month, as well as a second cap in Thursday’s 2-0 World Cup qualifying win over Peru.
That all in mind, it should come as no surprise to hear that European interest in Castillo’s signature is hotting up and The Sun’s report claims that an unnamed Premier League club – believed to be Swansea – has now lodged a £4 million bid to lure him across the Atlantic.
Getting hold of a work permit at this stage might be an issue but the fact that Colombia are high up in the FIFA rankings will help his cause if an appeal is needed.
Somewhat comparable to Swansea’s own Jefferson Montero, there is little doubt that he has the talent to succeed in the English top-flight and given his recent progress, the forthcoming transfer window could be the perfect time for him to make the move.
However, with Dallas currently sitting in the Western Conference’s top play-off spot with three regular-season games left, he still has the chance to help his side win both a Supporters’ Shield title and first-ever MLS Cup before he departs.