Ken Defends Grayson After Sheff Comments
But Bates, whose side take on Coventry at the Ricoh Arena on Saturday, said: "When you go on loan to a club you have a magnificent opportunity. It's a chance to get under the spotlight and the fact of the matter is he was very disappointing."
He added: "He didn't make an impact. It was down to him not Simon Grayson. If you want to be successful you look in the mirror and ask 'what could I have done?'
"No doubt he will be trying to prove us wrong and him right on Saturday. He has done well for them and I would say you're better letting your feet do the talking."