The 23-year-old suffered the problem on Saturday during his side's 42-0 defeat to Catalans Dragons in Super League.
Lomax will now undergo an operation before starting his rehabilitation.
"This is obviously a blow to the team but more importantly to Jonny. He has been in great form and has settled well into the full-back position," Saints coach Nathan Brown is quoted as saying by Sky Sports News.
"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."
St Helens continue their Super League campaign on Sunday when they welcome Castleford Tigers to Langtree Park.