The Newcastle striker, 27, told the People: "Only a fool would have said 'he won't come back as good as he left the pitch'.
"Obviously, if there were any fools out there then they'll get proved wrong. I've always been strong mentally - even when I went down on the pitch against Sweden.
"I've been to the best surgeon in the world and the best rehab. I've come back and I'd like to think that I'm every bit as good as when I left.
"The 10 days with England and the 10 days before that with Newcastle have gone really well.
"To start the season like this is obviously fantastic for both teams and myself."