The 18-year-old United States forward has long been touted as the first global superstar to emerge from Major League Soccer (MLS).

And as his arrival at Portela airport on Monday was greeted with a warm welcome from around 100 Benfica fans and reporters, his departure from America was also causing a stir.

Adu, who was captain of the US team at the Under-20 World Cup this year, has yet to officially complete his move from Real Salt Lake.

But Benfica, who finished third in the Portuguese top flight last season, have announced that they have paid $2.5million for Adu, who will pen what is believed to be a five-year deal once he has passed a medical.

"I am surprised with this reception," he said after touching down.

"I am very happy. Benfica are a big club."

Adu and fellow new signing, Argentinian Andres Diaz, trained with the team for the first time on Tuesday under the guidance of coach Fernando Santos.

Jason Kreis, the Real Salt Lake coach, believes the Ghana-born starlet has made the right decision to experience football abroad.

"I always thought in the right set up he could do very well in Europe," Kreis told PA Sport.

"He's a very technically gifted player, but physically he's quite small so he needs to be in a team and a league where they are going to let him play football.

"When I first heard about this deal and heard it was Benfica, I though that is the perfect scenario for him.

"The Portuguese league is a very technical league and as one of the top three or four teams Benfica can afford to let him play and let him grow, so I think he will get a lot of first team action.

"The club has shown it has got a great track record of developing young players."

The move will bring Adu's brief Real Salt Lake career to a close - he only joined the club in the winter in a trade from D.C. United, where he began his professional career as the youngest ever MLS player at 14 in November 2003.

Adu failed to truly shine for Salt Lake, who are struggling at the foot of the Western Conference with only one win and nine losses in 16 games.

A trial at Manchester United and informal talks with Chelsea failed to net Adu a deal in the Premier League, and he was also linked with Scottish Premier League champions Celtic before Benfica swooped.