The 27-year-old was out for two and a half months earlier this year after injuring an ankle against Wakefield in March and Saints coach Nathan Brown now fears he could be out for a similar length of time after damaging his other ankle after 17 minutes of Sunday's match.
"He's not so flush," Brown said. "They feel he may have done the exact same injury on the other foot, which is not great. The trendy word for it is syndesmosis. He's tested positive for two signs of it so it's not 100 per cent but he's certainly not going to be good because James Roby doesn't come off. He's a tough kid. If it is the same injury, it's a 10-weeker."
England coach Steve McNamara will be anxiously awaiting news of the injury when he arrives back on Monday from his three-week fact-finding trip to Australia.
Roby went straight into the England team for the game against the Exiles last month, just two games into his comeback for Saints, and could now be sidelined until the middle of September, just six weeks before the opening World Cup match.
The injury took some of the gloss off Saints' backs-to-the-wall victory at the Wish Communications Stadium, where they trailed 10-0 after 18 minutes after conceding early tries to Richard Owen and Jordan Tansey.
But Saints hit back through winger Tommy Makinson on 22 minutes and he added two more in the second half to complete his hat-trick and clinch an 11th consecutive win over Castleford.
Castleford coach Daryl Powell admitted his side had taken a step back after three consecutive wins had raised hopes of reaching the play-offs.
"I'm pretty disappointed," Powell said. "It was a good opportunity for us and at half-time I thought we were good value for where we were. I thought we should have been further in front. We dominated the game but they got us with a couple of sucker punches. Then we just didn't come out at the start of the second half and, once St Helens get you by the throat, they are pretty good.
"I expected there to be a few points today because of the conditions but it's not an excuse. Defensively we should have been better. Today was a step back. We've just got to keep working hard and front up in our next game."
- Sports & Recreation
- James Roby