St Helens full-back Lachlan Coote suffered a suspected medial collateral ligament knee injury in their opening round win over Salford.
The champions began their defence of the Super League title with a 48-8 victory against Salford but the home side lost Coote in the second half with a suspected medical ligament injury.
Saints coach Kristian Woolf said: “It looks like his MCL at the moment.
“We need to get it scanned, we won’t know the extent of it until the swelling goes down.”
In Coote’s absence, young gun Jack Welsby switched from the wing to full-back, where he had been playing in place of the injured Tommy Makinson.
The 18-year-old scored two tries and assisted one of Kevin Naiqama’s efforts.
Woolf added: “He’s a good kid, a good player.
“He showed plenty of glimpses of what he’s capable of tonight and remember he’s only 18.
“When Lachlan went down he certainly showed his class with a couple of terrific balls.”
Medial ligament injuries usually take two to four weeks to recover from, so Coote could be in doubt for the World Club Challenge against Sydney Roosters on February 22.