The 33-year-old Frenchman, now with Wigan believes Shepherd was "not good enough".
Sibierski Makes Amazing Attack On Shepherd!
Sibierski: "As a Newcastle fan, as I am, I was very happy when Shepherd left the club because he is the reason why they have not won anything for a long time.
"I was only there for a year but I could see what kind of chairman he was.
"I'm not saying this because I was upset with him when he changed his mind over me signing a new contract which I had agreed with Glenn Roeder.
"It's because as a chairman I never saw him once last year come and see us to have a meeting, or speak to us about results and aims we should have had.
"Compared to the other chairmen I have seen throughout my life and career, he was not a good chairman.
"I don't know him as a man, maybe he was a good man, but as a chairman he was simply not good enough.
"With Sam Allardyce as manager and the players he has brought to the club, and with the players already there, they are a really tough side now.
"I think they are going to win a trophy, and as a Newcastle fan I would be delighted if they won something this year.
"Newcastle fans deserve that, and I wouldn't be surprised if they finished top six because of the quality players and good manager."