The veteran midfielders swap West Coast teams, each hoping to be an impact on the playoff race.
Reo-Coker was an important player for the Whitecaps last season, but deemed surplus to requirements by the Canadian side after injuries and the aquisition and performance of Matias Laba in defensive midfield, and has only played in 12 of Vancouver's 23 games, often out of position.
"Nigel is a fantastic box-to-box player that has proven to be a leader in this league," said Chivas USA head coach Wilmer Cabrera in a statement released by the club. "He is an unbelievably versatile player. We feel he can help us with his defending as well as going forward in the attack.
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"We see his experience, leadership and great technique helping us in a lot of ways."
Vancouver was similarly pleased to welcome the 33-year-old Rosales, who has eight assists so far this season.
"We are excited to welcome Mauro to our club and Vancouver," said Whitecaps head coach Carl Robinson in his club's press release. "If you look at Mauro's history, he's constantly creating goals. He's experienced, bright, and has a super football brain. His delivery on crosses is fantastic, and I know he's a good guy. That's important in this locker room.
"We're very pleased to add a player of Mauro's caliber as we make our push for the playoffs."