I'm going to go out on a limb here and say that I don't believe this. I'm no expert, but I've read a lot about Donovon, particularly involving OSS ops in southern China and Indochina during and just after WWII, and have encountered nothing that would raise to the level of this scurrilous charge.
But it's always been easier to attack the dead than the living.
I've also read extensively on Donovan and the OSS, parrticularly with regards to the ETO. I agree, Donovan was a true patriot and idealist, but I suspect his idealism blinded him somewhat to the degree to which communist sympathizers infested the OSS (and the British SOE).
The book The Forgotten 500, which is primarily about OSS efforts to evacuate downed US fliers from the Balkans, also goes into extensive detail as to how much of eastern europe was ceded to the Soviets by their allies embedded in US the Allied intelligence and diplomatic circles.
Of course this doesn't prove whether or not Patton was assassinated, but the environment was such that the order could have gone out with Donovan being none the wiser.