"A scan has confirmed he has an unstable rib and there is a risk of further injury if Andy continues playing," said All Blacks doctor Deb Robinson.
Mathewson only made his Super 14 debut last year and has largely been employed as back up to All Black squad member Piri Weepu. The 22-year-old only experienced a regular run of games at provincial level during this season's Air New Zealand Cup when Weepu was moved to the fly-half position.
Mathewson is unlikely to play a part in Saturday's Test against Ireland at Croke Park but could feature in the encounter with Munster the following Tuesday.
With eight of his Wellington team-mates in the All Black squad, Mathewson's international inclusion should be made a little easier, although he admits the call to fly to the UK was slightly unexpected.
"I didn't get my hopes up because Andy just walked off. It looked like he just got a little knock on it," said Mathewson.
"But heaps of mates were asking 'are you ready to go'?
"I've been texting Cory Jane a bit and he says they're all friendly and make you feel welcome. So it should be quite easy to fit in. That helps because I'm quite a shy person. It's good to have a few people over there."
After facing Ireland and Munster, New Zealand take on Wales and England on November 22 and 29 respectively.
