Watford have sent Miguel Layun off on a season-long loan to FC Porto, who retain an option to buy for £4.4m at the end of the season.
The Mexico international arrived at Vicarage Road in January and opened his account for the club with a goal in the opening day 2-2 draw against Everton in the Premier League this season.
The 27-year-old will be relishing the opportunity of testing himself against the best in the Champions League, with the Portuguese side having been drawn in Group G alongside Chelsea, Dynamo Kiev and Maccabi Tel-Aviv.
Layun told the Porto website: "I am eager to start training, to be at the same level as my teammates and to fight for a spot in the starting team.
"The present is always the best moment, but I've definitely reached my full maturity. Emotionally, I'm in a great moment and all that will help me with the best performances on the pitch."