I'm by no means an insider of US soccer, but I've heard Donovan has mental and physical problems. In fact, looking at a recent picture, he seems to have become quite chubby.
There's no way a player in Donovan's obvious condition could help the team in Brazil. The climate will be brutal, extremely hot and humid. Professional players will be exhausted before they even step onto the pitch...and the older you are, the harder it will be to deal with the climate.
Bold move by Klinsmann, but judging from afar, it's the right one.