Guildford scored four tries against Canada in his first Cup appearance last weekend, but a day after he limped out of training, All Blacks assistant coach Steve Hansen said on Thursday that a scan has revealed a small tear in his hamstring.
McCaw was withdrawn from the pool finale with Canada because of the right foot injury which has bothered him all year and could require surgery after the tournament, although the All Blacks said he would have played if needed.
This week they need him, and Hansen confirmed the openside flanker will play.
"Now that we're down to the business end, whilst we're still in the tournament he'll be available," Hansen said at the team hotel.
How sure was he of McCaw getting through three more games, if New Zealand reaches the final?
"I'm confident," Hansen added.
The availability of McCaw, the All Blacks' inspirational leader and most capped player, has assumed even greater importance to the team's hopes of a first World Cup title in 24 years since they were shocked by the tournament-ending injury to their other superstar, flyhalf Dan Carter.
Carter, the leading pointscorer in international rugby, tore a tendon in his groin in training last Saturday.
Since February when McCaw underwent surgery to have a broken bone in his right foot repaired with metal pins, he has experienced discomfort from the friction of the metal. Last weekend he said the injury was "still niggly."
To help him, he trains in shoes instead of boots, and Hansen said the coaching staff don't let McCaw practice as hard as the other players.
"We don't get him involved in cleanouts, and physical, hard-contact stuff," he said.
Hansen said all the team which have for the quarterfinals have played at least half a dozen tests in the last two months, including warm-up matches, and all players will be playing through pain of some degree.
"Every player's going to be carrying niggles, so it's just how big a threshold they have for pain," he said.