The Major League Soccer team gave no hint of what Wednesday's announcement would be or whether it is linked to recent pressure from fans and media for the last-place Impact (3-12-5) to replace sporting director Nick De Santis.
The 24-year-old Duka has played in only six games this season. The American had four goals and three assists in 31 games last season while playing under coach Frank Klopas, who is now the Impact's head coach.
Gambian international Nyassi, 25, who was selected by Montreal from Colorado in the 2012 expansion draft, has been limited to two games this season due to a knee injury.
Duka has six goals and eight assists in 82 career MLS games. Nyassi has 15 goals in 120 career games.
Duka played his first three seasons for the Columbus Crew, from 2010-'12, before joining Chicago last season. He played for the U.S. in the U20 World Cup in Egypt in 2009 and was also part of the U23 squad.
He had five games of CONCACAF Champions League experience with Columbus.
The Impact begin Champions League play next week against FAS of El Salvador.