Reyna, 33, joined Manchester from Sunderland in 2003 but had not played since December. He has played 12 years in Europe.
The midfielder is expected to join the MLS's New York Red Bulls coached by Bruce Arena. Reyna was with the U.S. national team for eight years under Arena. He could join the team before the season begins in April.
Reyna spent a total of 14 years on the U.S. team and retired from international play after last summer's World Cup.