The 49-year-old announced on Thursday that Donovan was not in his plans for the finals in Brazil, despite holding 156 international caps and scoring 57 goals for his country.
Donovan's omission caused shock when it was revealed as the forward had been expected to be an integral player for the USA, in what would have been his fourth World Cup.
Klinsmann felt the 32-year-old was not fit enough to be included in his 23-man party, but revealed that delivering the news had been tough.
"That was certainly one of the toughest decision in my coaching career, to tell a player like him - with everything that he has done and everything he represents - to tell him that he is not part of the 23 right now because I just see some other players slightly ahead of him," the Germany coach said.
"He has been in that final 30 roster over the last 10 days and he did everything right, he was always positive, he took it in the best possible way.
"His disappointment is huge, I totally understand that, he took it very professionally because he is an outstanding professional player and he knows that I have the highest respect for him.
"But I have to make the decision as of today, I have to make the decision with what is good today for this group going to Brazil and I just think that the other guys right now are a little bit ahead of him. I told him that and he understands it, but obviously he is very disappointed.
"The door is not closed for them because we still have a couple of weeks left and, God forbid injuries do happen, you need to call them up and hope that they stay sharp, in shape, in rhythm and that they take it positively."
Klinsmann also omitted Terrence Boyd, Joe Corona, Maurice Edu, Brad Evans, Clarence Goodison and Michael Parkhurst from his final squad.
