But for the good of the All Blacks - and New Zealand - hopes are the captain will battle on.
"In general, resting stress fractures is the best option,'' one orthopaedic surgeon said yesterday.
Continuing to load the bone by heavy use could prevent healing.
"A stress fracture, by itself, is unlikely to put him off [playing] unless there is more progression ... He hasn't been limping, and that is a good sign.''
However, flanker Matt Todd joined an All Black training session in Auckland yesterday, sparking more fears over McCaw's fitness.
McCaw was wearing trainers rather than rugby boots at the session and left when reporters and photographers arrived.
Orthopaedic surgeons, none of them privy to McCaw's medical details, were only willing to speculate anonymously about the fractured fifth (outermost) metatarsal bone in the flanker's right foot.
A screw was placed in the injured bone in February. The aim of this is generally to share the load on the bone and, by compression, to help it heal.
Herald rugby writer Wynne Gray wrote last week that those close to the team had said McCaw would play the rest of the tournament.
"He will get by on treatment, painkillers and willpower, but may need another operation once the All Blacks end their campaign.''
But after McCaw played in Sunday's quarterfinal victory over Argentina, All Black doctor Deb Robinson said there were no plans yet for surgery after the tournament.
McCaw has referred to the injury as "niggly at times''.
Surgeons told the Herald the screw had probably not come loose.
"Anything is possible but that's relatively unlikely,'' said one.
Another - this one an Australian - said: "If he puts a lot of weight on that foot and gets a sudden, sharp stab of pain, it might give way at a critical moment.
"From the game point of view, it's difficult. From the foot point of view, if there's some healing taking place and he reinjures it, it's back to square one.''
He said the healing might be impaired if the fracture was in a part of the bone that is known to have a poor blood supply.
Everyday walking might be painless for McCaw, but "if he twists his foot it will be pretty sore, like a bad ankle sprain'', the surgeon said. Oral painkillers were commonly used, but sports physicians sometimes gave players injections of local anaesthetic.
- Martin Johnston of the New Zealand Herald