The cable, sent by the American Embassy in Berlin, dates from 31 December 2009, just weeks before Oettinger's confirmation hearing in the European Parliament.
It says the German Chancellor's move to nominate Oettinger "was not the promotion of a valued colleague as [Angela] Merkel's allies sought to portray it".
Rather, Merkel was seeking "primarily to remove an unloved lame duck from an important CDU bastion". Prior to his nomination, Oettinger was minister-president of Baden-Württemberg, one of Germany's most powerful regions.
Oettinger's appointment was met with great surprise at the time in Germany, with the socialist opposition saying the European Commission had become "the dumping ground of a failed minister-president that Merkel is trying to get rid of".
More generally, the leaked diplomatic cable is unflattering for Germany's choice of commissioners, saying Berlin has "a time-honoured tradition of sending unwanted politicians to the EU Commission".
"Oettinger is noted for a lacklustre public speaking style, and some commentators have asserted that Merkel, who has often stood out at EU meetings, wanted to appoint a German Commissioner who would not outshine her."
Oettinger's spokesperson, Marlene Holzner, declined to comment, saying it was the Commission's policy not to react to WikiLeaks revelations.
An EU lobbyist following energy matters in Brussels said Oettinger's portrayal in the leaked cable was "blunt but not inaccurate".
"The assessment that he was shoved out of Germany seems to stand up," the lobbyist said. "He was one of the senior politicians in Germany that Merkel got out of the way, so to speak."
Lack of experience on energy
The cable, titled 'Lame Duck German Governor Kicked Upstairs as New Energy Commissioner in Brussels,' also calls into question the commissioner's policy experience, noting that "his background in energy appears limited to public support for nuclear power and the extension of power plant operating licences".
"Oettinger has not expressed interest to date in energy issues other than nuclear," the cable notes, adding that "his pro-nuclear stance is in stark contrast to many other German politicians, who support the phase out of nuclear power".
"This endears him to the EU, which is focused on diversification of energy sources and does not exclude nuclear energy."
In fact, Oettinger won plaudits during his confirmation hearing in the European Parliament in January, when he said he wanted to work "towards a Europeanisation of our energy policy".
And the cable does admit that "Oettinger is said by industry sources in Baden-Württemberg to be an efficient behind-the-scenes negotiator".
But questions were raised at the time as to whether the Baden-Württemberg premier was committed to putting aside the interests of the nuclear power utilities he had worked closely with in his previous job.
"Nuclear power plants supply most of BW's energy while renewable energy and fossil fuels do not play a significant role," the cable notes.
'Tendency to put his foot in his mouth'
The leaked diplomatic telegram also describes Commissioner Oettinger as prone to making blunders. "In addition to being a poor public speaker, he has a tendency to put his foot in his mouth," reads the report.
In March this year, Oettinger surprised observers by suggesting the EU-favoured Nabucco gas pipeline project could be delayed to 2018. The Vienna-based consortium behind Nabucco later issued a statement to say the project was still on track.
In July, the German commissioner also raised eyebrows in Brussels by expressing his support for an EU-wide harmonised scheme for renewable energy. The statement was later corrected by his own services, who have long argued that national schemes are better suited to support renewables.
But others in Brussels are not so quick to disparage him. "Commissioner Oettinger has announced he will come up with important concrete steps on infrastructure and energy efficiency next year," says another EU lobbyist who specialises on energy issues.
"If he presents a well thought-through strategy and secures support for concrete policies he will prove a valuable part of the Commission."