But the Boro chairman is determined to claw back £10m compensation which the club say they are owed following the move.
"The club will not rest in its pursuit of the truth and for proper recompense," said Gibson.
"We lost a very, very prominent player through illegal means. That happened two or three weeks before the start of the season.
"And when I say illegal means, that's not my verdict, that's the verdict of the Premier League.
"The Premier League found Liverpool guilty of illegal action in taking Christian Ziege to Liverpool at the expense of Middlesbrough, and that had a terrible effect at the club."