Widdop limped from the field a minute before half-time in Tuesday's ANZAC Day match between St George Illawarra and Sydney Roosters at the Allianz Stadium in Sydney with what appeared to be a medial ligament tear.
Touchline reporter Brad Fittler told television viewers he thought it was a grade 1 or 2 tear which would make the Dragons captain a major doubt for the game against Samoa in Sydney on Saturday 6 May.
Widdop, named on Monday in Bennett's 20-man squad, would have been an automatic selection after making an impressive start to the NRL season with the Dragons, 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 through his grandmother - could also be an option.
Alternatively, Bennett could add a second hooker to his 20-man squad, with Warrington's Daryl Clark the favourite to join Canberra's Josh Hodgson.
The bulk of the England squad will fly out on Saturday, with the players involved in the Castleford-Wigan game that evening catching a flight the following day.