In a new book, McCaw writes that he couldn't even walk properly during the tournament.
"I've got to be careful to mask the worst effects of the injury not just from the media but also from all the people constantly coming and going from the hotel. Not to mention the team and the coaches."
» Read an extract from Richie McCaw: The Open Side.
He suspected his foot was broken but never had the X-ray which would have confirmed it.
Instead he got through the World Cup on painkillers, adrenaline and willpower.
"If we know for sure it's broken, then it's going to be much more difficult for [McCaw and team doctor Deb Robinson] and everyone around us to keep me on the field. Because whatever the ramifications, I'm going to keep playing on it as long as I can stand up and do my job."
Co-written with Greg McGee, The Open Side details McCaw's rise from age group player to All Blacks legend.
An extract published today reveals the star flanker's injury woes during the 2011 Rugby World Cup and his empathy when Dan Carter was crocked out of the tournament.
McCaw writes of sitting with a dejected Carter, after a groin injury cruelly robbed his mate of a shot at a Rugby World Cup final.
"Dan asks questions to which there are no answers. ‘Why? Why me? How did this happen? I was just kicking the ball, the same ball we used in the Super 15, the same routine I've done a thousand times.' There's bugger all to say, except, ‘Oh, mate, I have no idea. I'm so sorry.' Then he tells me that he used to think everything happened for a reason but he can't believe that any more."
In the book, McCaw accepts part of the blame for the agonising quarterfinal defeat against France in 2007.
"The guts of it is that in that moment, our leadership group failed under pressure. That includes me. Rather than saying, ‘Have a pot if it's on,' I should have been more directive."
However, he reveals the 2007 All Blacks didn't even have a drop-kick play up their sleeve, nor had they considered the possibility of a tight finish.
"We believed that we were good enough to go out and play well . . . it was a failure of imagination as much as anything."
McCaw doesn't blame referee Wayne Barnes for the loss, but questions why an inexperienced whistleblower was given the game.
"This was Barnes' biggest game by far. On the big stage, an inexperienced referee is likely to become so afraid of making a mistake that he stops making any decisions at all.
"By the end of it, I thought Barnes was frozen with fear and wouldn't make any big calls."
Four years later at Eden Park, when the final whistle blew the All Blacks gained redemption, and McCaw was overwhelmed with relief.
"I bend over, hands on knees. Then sink to one knee. We've won. I should be happy. All I feel is relief. It's finished. I can stop. I don't have to do this any more."
He was so drained he lifted the Webb Ellis Cup "on auto-pilot".
"The moment I've been waiting four years for. I thought I'd feel more. It's like I'm seeing it all through someone else's eyes. The welling emotion of the crowd rolling over me, too mentally and physically shot to really respond."