Pires is off contract in June and confirmed this week he intends to play on next season - possibly in the 'States.
"The league [MLS] is definitely interested, there is currently a large number of teams on board and Philadelphia is one of them," Pires' agent Martin Djemmal told philly.com. "We were first contacted by the team in February, and there has been back and forth since.
"It's a matter of the transfer window in July, but right now, I'd say it's 50-50 at this point [that Pires will join the MLS].
"He's a big player and a lot of teams are interested."
And Union coach Peter Nowak has confirmed his interest: "There have been confirmation on both sides, I will go on record with that
"I believe that a player like Pires with a great pedigree and a great background could be a role model for these young players. But I don't like the name designated player, I think the term is artificial. Either you are a good player or a bad player."