There's not much point in staging a coup if you don't influence who is placed in power in the aftermath. Of course in order for a puppet government to be effective, they can't be perceived as such. You wouldn't want the natives to get restless would you?
The evidence that the U.S. was behind the toppling of the Ukrainian government early this year is so overwhelming at this point that the subject really isn't up for debate, however initially it was unclear how the election of Petro Poroshenko fit in. The ecstatic response by Washington when he was declared the winner, and their unbending support in spite of his ongoing military assault against civilians in the east, made it clear that he was the chosen one, but the paper trail wasn't immediately obvious.
As it turns out, the evidence that Poroshenko is in the pocket of the U.S. State Department has been available all this time, you just had to know where to find it. In a classified diplomatic cable from 2006 released by Wikileaks.org, U.S. officials refer to Poroshenko as "Our Ukraine (OU) insider Petro Poroshenko".
A separate cable also released by Wikileaks makes it clear that the U.S. government was considered Poroshenko corrupt.
"Poroshenko was tainted by credible corruption allegations, but wielded significant influence within OU; Poroshenko's price had to be paid."
The U.S. government knew Poroshenko was dirty, but he was influential, and arguably their most dependable mole.
Perhaps the most interesting revelation comes from a 2009 cable where Poroshenko told then Secretary of State Hillary Clinton he supported "the opening of a U.S. diplomatic presence in Crimea" and "He emphasized the importance of Crimea, and said that having U.S. representation there would be useful for Ukraine." Poroshenko's role as an informant for the U.S. government continued in cables in 2010 as well.
Reading through the cables, I have to wonder if Poroshenko was actually breaking Ukrainian law by sharing the kind of strategic information that he did. Considering that this information was certainly used when planning the coup against Yanukovich, one could argue that he committed treason.
Poroshenko, however, isn't the only Ukrainian politician mentioned. For example, the cables mention the scandal surrounding Oleksandr Turchynov's destruction of SBU documents tying Julia Tymoshenko to organized crime, and note that the accusation that Tymoshenko wanted Turchynov get the Interior Minister position so that she could gather damaging information on her enemies. The cable refers to this accusation as "not farfetched". Turchynov went on to be installed as the acting president of Ukraine in the provisional government.
In order to grasp the extent of the U.S. government's tinkering in Ukraine it is worth reading the documents for yourself.