Klinsmann raised eyebrows earlier this week with comments about Bradley, who left Roma for Toronto this season, and Seattle Sounders striker Clint Dempsey, who quit Tottenham for MLS.
"It's going to be very difficult to keep the same level that they experienced at the places where they were,'' Klinsmann said Monday. "It's just reality. It's just being honest.''
The U.S. coach also said Bradley, after a frustrating season in Toronto that seems destined to end before the playoffs begin, "has to prove that he hasn't lost a bit.''
An incensed MLS commissioner Don Garber responded via conference call, rejecting Klinsmann's comments.
The 27-year-old Bradley, who has won 91 caps for the U.S. while playing in MLS, England, Germany, Italy and the Netherlands, seemed to see the controversy as motivation.
"I've certainly had moments in my career where people have challenged me, said that I've not been good enough, said that I'll never be good enough. But not only has that never stopped me, it continues to motivate me and fuel me even more," he said after practice Friday.
"And so for me, it's all very simple. My record when I've played for the national team and my commitment and contribution is there for anybody to look at and take from it what they want."
Bradley says he will continue to challenge himself to become "a better player, a better leader for as long as I have the opportunity to."
"Because playing for your country and playing for (the) national team, there's no greater honour," he added. "At the end of it, people can look at that and draw from that what they want. But I sleep very easy at night knowing everything that I have given and everything that I've put into it. And I'll continue to do so until they don't let me any more."
Toronto FC coach Greg Vanney, a former U.S. international who played both in MLS and France, called Klinsmann's statement about MLS "harsh."
"I think this league is extremely challenging, but maybe in different ways," he said.
Star players in MLS have to take ownership of their teams unlike in Europe, not to mention cope with the physical play and extensive travel.
"Michael shows up every day and he puts in the work," said Vanney. "And he's an honest worker.
"So I think if you're the type of person who comes back to this league and you're on vacation, I think it (playing in MLS) can hurt you. If you're the type of person like Michael and what I hear of the other guys that come back, this league it just provides you with a different challenge, that's all.
"I think it can keep you sharp and it can keep you ready for any international play."
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