The 28-year-old has a fractured left cheekbone, it has been confirmed, and will be out for a minimum of six weeks.
He sustained the injury in a challenge by Wigan hooker Terry Newton during last Friday?s 38-12 win at the JJB Stadium and will undergo reconstructive surgery later this week.
He will be the fifth player to withdraw from Great Britain coach Brian Noble?s initial squad, which has been depleted by the loss of Paul Sculthorpe, Kris Radlinski, Paul King and Newton.
Saints suffered a double blow with confirmation that Australian winger Darren Albert also sustained a cheekbone fracture in the same match will miss the club?s bid to reach the October 15 Grand Final.
The 28-year-old Albert has signed to play for Cronulla next season and, therefore, has played his last match for St Helens and ends his British career with 91 tries from 124 appearances.
?It?s disappointing for both players because this group is pretty tight and they do feel for each other,? said coach Daniel Anderson. ?It was quite sombre on Friday night but there is a resilience about the club and we have some boys here who will step up.
?You do want your big guns for the play-offs. We?ll just have to knuckle down and work as a unit.?
Carl Jones, a consultant facial surgeon, said: ?Sean Long has a fracture of the left cheekbone with involvement of the eye socket, while Darren Albert has a fracture of the right cheekbone with possible involvement of the eye socket.?
St Helens, who clinched the League leaders? trophy with their win at Wigan, are already without skipper Paul Sculthorpe and prop Mark Edmondson, leaving Anderson with just 18 fit players for Saturday?s final match of the regular season against Bradford.
Saints are unable to bring more players in because the signing deadline passed earlier this month but Anderson is loathe to throw a host of youngsters in at the deep end.
?There?s no avenue except for the players we have in the club at this moment,? he said. ?I?m loathe to put in kids against a Bradford team who are full of seasoned Super League players. We?ll have as strong a team as we can put out.?
Saints are set to welcome back New Zealand loose forward Vinnie Anderson, who resumed training on Monday after missing the last two games with a leg injury, and Keith Mason and Ian Hardman have been added to the squad.
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Thanks to www.superleague.co.uk for this news.Like