Valdes, 29, went on loan to Colombian club Independiente Santa Fe in January 2013. He had joined the Union from the same club 2 years earlier.
"I think he's a top center back in MLS," Curtin said. "If we can add that piece to us, that's a huge piece."
Valdes started and played all 90 minutes in Colombia's 4-1 victory over Japan in Group C play on June 24. That will surely increase his stock when his country finishes World Cup play. Colombia plays Brazil on Friday.
The addition of Valdes could solidify the Union's backline. The one-time Union captain was an MLS All-Star in 2012, the year the All-Star Game was played at PPL Park.
"The huge factor is he's a known commodity," Curtin said. "Listen, there are other center backs in the World Cup that have expressed a real interest to come here, like legit starting center backs in the World Cup that would like to be here. However, Carlos knows the league. He's our player still. It's a quick transition."
Several variables are involved in a potential return for Valdes, Curtin said, among them being the fact that other clubs will seek the player's services.
"We think we've made a very positive offer, an aggressive offer to get him back here in terms of upping the ante down the road with him," Curtin said. "So he's a guy we think for the long term we want him in a Philadelphia Union jersey."
Midfielder Vincent Nogueira returned to training yesterday after sitting out Saturday's win at New England and the last Open Cup match with a groin injury. Jim Curtin said Nogueira reported no pain or issues, and the manager deeming it a step in the right direction. "Vincent's obviously one of the best midfielders in this league," he said, "so anytime you can add him into a group that's confident, it's a good thing."
On Twitter: @jakemkaplan