Halifax-born Widdop, 24, was initially ruled out for the season after suffering the injury in Melbourne Storm's match against Gold Coast on June 24 but, after making a quick recovery, he will play for feeder club Easts Tigers against Mackay Cutters.
Centre Sa'u, 26, who will also line up for Easts as he continues his comeback from a seven-week lay-off with a foot injury, is delighted that Widdop will get the chance to put himself in contention for the World Cup.
"Before he got injured, he was in some great form," said Sa'u, who is also aiming to gain match fitness ahead of the World Cup.
The Auckland-born centre made eight appearances for the Kiwis from 2009-10 but switched his allegiance to Samoa earlier this year and is in line to make his first appearance at the Salford City Stadium next month.
The Samoans will warm up for the World Cup with a match against England Knights, which will be played as a curtain-raiser to England's friendly against Italy at Barton on Saturday, September 19.
Sa'u is one of 12 new signings for the renamed Red Devils, having signed a two-year contract the day after Widdop dislocated his hip, and he is looking forward to joining the revolution under owner Marwan Koukash and former Great Britain coach Brian Noble.
England internationals Rangi Chase and Gareth Hock are among the host of new faces set to run out at the Salford City Stadium next season.
"It's exciting times for the club," Sa'u said. "Obviously they've bought well.
"It's a new chapter for the club and something I want to be part of. I'm looking forward to getting over there."