The fullback eventually moved to Tottenham after rebuilding his career at Burnley.
Cassell told the Daily Mail: "We should have kept Kieran and loaned him but the club had Micah Richards and Pablo Zabaleta at the time so he was sold to Burnley. But Kieran struck the ball like a Premier League player as a nine-year-old. You look for things in young players that they will never lose. And that was Kieran's.
"He is the best technical full back we have and the best crosser of the ball since Beckham. I am not blaming anyone for letting him go. But I suspected it would be our loss and not his."