Watson-Siriboe will be reunited with coach Jason Kreis after NYCFC sent a 2016 fourth-round draft pick to Real Salt Lake for the 27-year-old Monday.
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The 6-foot-3 center back played 20 games — including one playoff start — under Kreis for RSL from 2012 to 2013 but had not made an appearance this season.
"He is a player that was great to coach and I'm confident he will be able to contribute to the strength and shape of our back line as we continue to build towards our first game in March 2015," Kreis said in a news release. "Kwame is the type of imposing central defender that is difficult to play against, adding him to the squad at this early point in our roster building process is fantastic."
Watson-Siriboe joins David Villa, Frank Lampard, Josh Saunders, Jeb Brovsky and Andrew Jacobson on NYCFC's roster.