The Mexico global signed for Porto yesterday on a season-long loan deal – with the option to sign permanently for €6 million.
The Mexican global said he hoped to break into the first team at Porto, who won the Champions League in 2003-04.
Layun played 17 times Watford after completing a permanent switch from Grenada on a four-and-a-half-year deal in January 2015.
Layun appeared in three of Watford’s four Premier League matches in 2015-16, scoring in their 2-2 draw with Everton on the opening day.
Watford will not want to sell the player, he has become a regular in the first-team.
Porto sign FC Twente winger Jesus Corona for an undisclosed fee. And his move to Portuguese giants is seen as a big step in his budding career.
The club now has two many foreign players and they have taken Daniel Pudil and Fernando Forestieri out of the club, sending them both to Sheffield Wednesday with Matej Vydra also set to depart.