Scans have confirmed that Ennor ruptured the anterior cruciate ligament in his right knee during the North v South match in Wellington at the weekend.
Ennor, 23, who was named in the All Blacks squad on Sunday, will now see a specialist over the next few days, is expected to undergo surgery and be out of rugby for up to nine months.
All Blacks Head Coach Ian Foster said: "It's hugely disappointing for Braydon, but he is a strong young man and we'll be supporting him as he goes through his surgery and rehab over the next few months."
Foster said the All Blacks selectors would take their time to name a replacement and would confirm that in due course once the All Blacks Test programme was finalised.
Ennor was one of four midfielders selected in Foster's first All Blacks squad with Jack Goodhue, Rieko Ioane and Anton Lienert-Brown.
He has played one game for the All Blacks against Argentina in July 2019.
Ennor's injury will force him to miss Canterbury's Mitre 10 Cup campaign and the majority of any Super Rugby in 2021.