Rooney was sent off after appearing to stamp on the groin of Portugal defender Ricardo Carvalho in the 62nd minute of the quarter-final in Gelsenkirchen two weeks ago.
England went on to lose the match on penalties, after extra time ended in a 0-0 draw.
"I'll go to my grave and still maintain it was a complete accident," Rooney said in extracts from his autobiography published in the Mail on Sunday.
"When I fell I had my back to the player. I couldn't see him, or where I was putting my foot.
"If it had been a definite stamp meant to harm him, the fella would still be in hospital to this day. But he was up on his feet in minutes, no worse for wear.
"It was all the sort of thing that happens when you are fighting hard for the ball," added Rooney, who was banned for two matches and fined 5,000 Swiss francs ($4,044) by FIFA's disciplinary committee for the incident.
Rooney said he was "disappointed" that Portugal's Cristiano Ronaldo, his team mate at Manchester United, had rushed to the referee after the incident, apparently to demand that a red card be shown.
Ronaldo was seen winking at his bench as Rooney walked off.
However, Rooney said his anger with Ronaldo did not last.
"On the coach, on the way home after the game, I sent a text to Ronny (Ronaldo). I told him to forget about what happened. I wasn't blaming him for interfering.
"Then I wished him and Portugal good luck in the semis and hoped they got to the final. And I meant it."
Portugal lost 1-0 to France in the semi-finals.
Newspapers have linked Ronaldo with a move to Real Madrid but Manchester United said this week that they expected him to honour his contract, which runs until 2010.
($1 = 1.23 Swiss francs)
