The 25-year-old, who departed Bremen in the summer, had been expected to move to the Premier League and has admitted his surprise at the snub from the Potters.
"I won't do that again. I played in two friendlies and the coach (Tony Pulis) told me that I had been one of the best - but he could not buy me because he had to sell two defenders first. That's when I asked him if he was joking with me. England is just strange," he told kreiszeitung.de.
Boenisch represented Poland at Euro 2012.