Dempsey switched from Tottenham Hotspur to Seattle Sounders, while Bradley moved from Serie A side Roma to Toronto FC earlier this year.
When asked if he was concerned about players maintaining their levels in the MLS, Klinsmann told reporters: "There's nothing I can do about it.
"I made it clear with Clint's move back and [Bradley's] move back that it's going to be very difficult for them to keep that same level that they experienced at the places where they were. It's just reality. It's just being honest."