Kiwi international Soliola, 23, has had a difficult start to his Saints career, and he picked up a ruptured patella ligament injury in his knee during training on Wednesday.
Saints Head Coach Mick Potter said: “This is a real blow for the team and we are naturally disappointed for Sia. He will now undergo a long rehab programme and we are all behind him he begins that process.
“As with last season, injuries also present the opportunity for younger players in the squad to step up and we will once again utilising the versatility of our squad.”
Soliola joined Saints last year and made his debut against Hull FC before missing five games with ankle ligament problems. He returned in Saints’ win over Warrington in the second row.
Meanwhile, Wellens is out for a month with a torn thigh muscle, which forced him to go off during last Friday’s 28-18 win over the Wolves.