Sibierski saluted the fans he leaves behind, but accused Newcastle's chairman of reneging on a two-year deal the striker had agreed with former boss Glenn Roeder.
The 32-year-old, who stepped in for the injured Michael Owen, was furious that Shepherd, now under pressure to quit himself, would only offer a one-year contract.
He said: "I agreed with Glenn Roeder last February about a new deal. It was a good deal. Now Glenn is gone and when my agent spoke to the chairman, it was not the same deal.
"It was a lack of consideration from the chairman. He never explained it to me, just spoke to my agent.
"I'm disappointed because I've left the Newcastle fans, with whom I had a great relationship. The takeover is a good thing for them. They deserve to win a trophy."