The 23-year-old will require surgery on the injury, as well as lengthy period of rehabilitation.
“This is obviously a blow to the team but more importantly to Jonny,” said Saints coach Nathan Brown.
“He has been in great form and has settled well into the full back position.
“He will now begin his road to recovery led by our medical staff and we wish him well.
“Jonny has a strong character and we’re sure he will come back and prove once again to be a pivotal player for us.”