Sporting Kansas City Front Office
Gedion Zelalem joined Arsenal's academy at age 15.
The trade comes shortly after Opara asked Sporting KC for a raise.
Toronto FC's Sebastian Giovinco, the Columbus Crew's Kei Kamara and Sporting Kansas City's Benny Feilhaber are finalists for the MLS Most Valuable Player award.
Rafael van der Vaart has said he "can imagine playing in America" amid interest from Sporting Kansas City.
MLS commissioner Don Garber responded to criticism of his league made by Jürgen Klinsmann.