It is a major blow for Lions head coach Warren Gatland who names his squad next week.
North was hurt during Ospreys’ victory over Cardiff Blues last weekend, and he tweeted: “Sport can be cruel.
“We all know the risks when we take the field. Unfortunately, I ruptured my ACL (anterior cruciate ligament) on Saturday and will need surgery next week. Heartbroken is an understatement.”
The normal recovery period for such an injury is six-nine months.
Gatland names his 36-man squad and captain on May 6, but a revised itinerary for South Africa made necessary by the coronavirus pandemic has yet to be published.
Before leaving for South Africa the Lions will play a warm-up match against Japan at Murrayfield on June 26.