In a column for the Sunday Mirror, Lukic wrote: "Petr was among the world's best in his first couple of years at Chelsea. He was truly outstanding and he hasn't become a bad keeper overnight either.
"He's suffered a loss of form and I think the incident in which he suffered a fractured skull at Reading back in 2006 has had a bearing on his performances since then.
"I sense there's something at the back of his mind niggling away. There are times when he could be more committed in dealing with a situation. There seems to be a reluctance on his part to go as whole-heartedly for the ball.
"He doesn't look as confident in his job."