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The former wonderkid made his first foray into Europe with the Portuguese outfit back in 2007 in a £1 million deal from Real Salt Lake.
However, the man once dubbed "the new Pele" played just 14 times and was sent on four different loans away from the Estadio da Luz.
The first was at Ligue One outfit AS Monaco, before spells at Belenenses, Aris and Caykur Rizespor followed. Adu would return to the United States with Philadelphia Union on a permanent basis in 2011.
But having seen Di Maria progress into an important player for Benfica and later reach world-class level following his Real Madrid move, Adu believes his departure from Benfica is where the point where things started to go wrong.
"The biggest mistake I made in my career was leaving Benfica on loan to Monaco," Adu told The Blue Wire Podcast as part of an in-depth interview.
"I say it from the heart. It was one of those decisions that, if I could take it again, I wouldn't make it.
"I had three coaches in a year in Benfica. The club was in such a dysfunctional moment that I just wanted to get out of there and go the other way as soon as possible.
"But it turned out to be the worst decision. I joined the club at the same time as Di María. In the first year, I was better than him.
"I played better than him, but I decided to leave for Monaco on loan. And Di María stayed at Benfica. And guess what?
"He had the chance to play with a coach who later came and became a starter. A year or two later he went to Real Madrid. And I ended up being loaned to a secondary team.
"I made the worst possible decision for my career. It is my biggest regret."
The first pick in the MLS draft and the league's youngest goalscorer at just 14 in 2004, Adu became viewed as one of the most promising youngsters in world football and a trial with Manchester United in November 2006 followed.
But fast forward to today and Adu's most recent move was the 15th of career as he signed with Osterlen FF in the Swedish third tier in October.
Prior to that, he had been out of football for nearly two years after leaving Las Vegas Lights in 2018.
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