The teenage Honduras international will join fellow MLS alumnus Sacha Kljestan in the EU capital.
Per team and league policy, the details of the deal were not disclosed.
“Andy is an extremely hard player to replace with our group but I'm excited for him and the opportunity he has in front of him,” said D.C. United head coach Ben Olsen. “Watching Andy grow and develop as a player has been a very rewarding process for the club, our staff, and me personally, and we wish him the best in Europe.”
Najar, 19, was the 2010 MLS Rookie of the Year, and becomes the League's first Homegrown Player to move to Europe on a permanent deal.
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