The Halifax-born player damaged knee ligaments in St George Illawarra's NRL defeat to Sydney Roosters on Tuesday, less than 24 hours after being named in England coach Wayne Bennett's 20-man squad for the international against Samoa in Sydney on May 6.
St George Illawarra's head of athletic performance Nathan Pickworth told the club's website: "Scans have shown that Gareth has some significant damage to the medial collateral ligament in his left knee so for the next two weeks he will be in a brace.
"After those two weeks, we'll have a good indication of his return-to-play time-frame but hopefully it will be somewhere around the six-week mark."
Widdop, who has been in good form for the Dragons so far this season, would have been an automatic selection against Samoa, lining up in the halves alongside either Castleford's Luke Gale or Wigan's George Williams.
They were the only half-backs named in the squad but Bennett still has the option to call up St Helens scrum-half Matty Smith or Warrington duo Kevin Brown and Stefan Ratchford from his England-based 30-man elite training squad.
Canberra half-back Blake Austin - who qualifies for England through his grandmother - could also be an option, alongside Australia-born forwards Chris McQueen and Chris Heighington.
Austin said: "If I had the chance to make my grandmother proud and it was a special moment for her, then I would definitely put my hand up.
"She's one of the toughest women I know and she was pulled away from her parents in the war-time and sent to Australia."
However, Austin did admit that he didn't want to "offend anybody" if it meant switching his allegiance from the Kangaroos to the English.
"That'd be something you consider further down the track. I know a lot of players come out and voice their opinion on things like that and I wouldn't want to offend anybody," he said.
"Obviously it means a lot more to those guys than it would to me and Origin is on the top of my goals list but if they reached out to me then it's something I would think about for sure."
