The former All-Star center back, who represented quarterfinalist Colombia at this summer's World Cup, has returned to the Union after spending the past year and a half on loan in South America, the MLS side announced Monday.
"When I left from Philly, I always was thinking to come back one day," Valdes said at a news conference. "I think this is my second family and I love the fans, I love the team, I love the people in Philadelphia and I love my teammates. As soon as the World Cup finished, I started thinking again about coming back to Philadelphia."
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Valdes started 65 games for the Union from 2011 to 2012 before returning to his former club, Colombian side Santa Fe, for a loan stint in 2013. He then joined San Lorenzo in Argentina on loan earlier this year.
The 29-year-old made one appearance for Colombia in Brazil, starting a 4-1 win over Japan in the group stage finale.
Center back has been a position of uncertainty for the Union this season, with the spot filled at times by natural midfielders Maurice Edu and Amobi Okugo, converted forward Aaron Wheeler, regular right back Sheanon Williams, and young newcomers Austin Berry and Ethan White.
"He's a guy who is familiar with the league, who is coming off of a World Cup for what I thought was the best team in the World Cup," Union interim coach Jim Curtin said. "We know he's grown a ton as a player. That experience and leadership he'll bring to our locker room is invaluable. He's a guy that can slide right in and be a top center back in our league right away."