He told the Daily Mail: "Confidence plays a massive part. When I first joined Everton, I wasn't sure if I should or shouldn't be here.
"But now I believe I should be here. Thanks to the manager and his own experience as a centre-half and the players I have around me, I think I've moved up a notch or two in every department.
"I'm a better defender in every department than the one who came here.
"I was happy with where I was at Sheffield United. I went there at 16 as a trainee and I had a great time but I might not have been under enough pressure. I might not have developed as fast as I perhaps could have."