The Australian, signed on a short-term contract from St George Illawarra, helped the Broncos reach the last four of the knock-out competition with a try, two goals and a drop goal in their 29-10 victory over Sheffield Eagles.
Rea says Soward will miss the clash with the Vikings due to a back injury.
“We genuinely have some bad knocks,” the Broncos boss said. “We’re not resting anybody this week.
“He’s done a real lot of travelling and he’s stiffened up during the week.
“Given his workload and what he’s been asked to come in and do, his back was untrainable today (Wednesday) so there was no way, given the number of miles he’s travelled, that he could compete in this game safely.”
Rea has added on-loan forward Jacob Fairbank, who was ineligible to play against the Eagles, to his 19-man squad, while centre Ryan Shaw, who has joined on loan from Swinton, is set to be handed his debut.
Widnes, meanwhile, saw their Wembley dreams dashed on Monday night when they went down 48-4 to Wigan.
Widnes 19-man squad: Allen, K. Brown, Cahill, Clarke, Craven, Gerrard, Gore, Hanbury, Isa, Joseph, Kavanagh, Lawton, Leuluai, Marsh, Mellor, O’Carroll, Owens, Phelps, Winterstein.
London 19-man squad: Bryant, Colbon, Dollapi, Fairbank, Fisher, Grady, Hurst, Kaufusi, Krasniqi, Lee, Lloyd, McMeeken, Melling, O’Callaghan, Shaw, Rodney, Woodburn-Hall, Wheeldon, Wicks.
Prediction: Widnes have shown they can attack against anyone and their defence should be able to keep London out. Home win.
Last meeting – London 14, Widnes 28 (SLR1, 3/2/13)
Super League summary:
Widnes won 4
London Broncos won 10
Widnes highest score: 38-30 (H, 2012) (Widest margin: 28-14, A, 2013)
London Broncos highest score: 68-10 (H, 2005) (Widest margin: 66-8, H, 2005; 68-10, H, 2005)