“Ricardo failed his medical. Everyone knows he has got a knee problem and Sunderland knew that in advance," Pompey chief executive Peter Storrie said.
“We had structured a pay-as-you-play deal that was the same kind of deal as he had here.
"His knee might need an operation at some stage, but it could be next week or it could be in the next five years. Everyone knew that was the case.
“It is surprising that this has happened after Sunderland knew the facts anyway,” he added. “It is frustrating, especially for Ricardo.
“I am sure someone will take him because he has proved with us that his knee is OK because he appeared in more games for us last season than almost anyone.”