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The Welsh will be vying with England and Australia to top Pool A, which also includes Fiji and Uruguay.
Injuries to backs Leigh Halfpenny, Rhys Webb and Eli Walker have disrupted preparations but Marshall refuses to discount Warren Gatland's side.
"They've got an opportunity to be that silent assassin team because they've slightly been written off," he said.
Full-back Halfpenny and scrum-half Webb, both 26, are out of the tournament after suffering knee and foot injuries respectively in the warm-up win over Italy.
Winger Walker is also out with a hamstring problem.
"What a great opportunity for Warren Gatland and [captain] Sam Warburton to get together and go 'they've kind of written us off, let's sneak into Twickenham and ambush England and let's put the sword to Australia," former Ospreys scrum-half Marshall told Sport Wales.
"I'm very much watching with great interest as to Wales' performances in the pool stages as they could upset and they could qualify."
Watch the full Justin Marshall interview on Sport Wales' Rugby World Cup special on Thursday; BBC Two Wales, 19:00 BST. Available on iPlayer for seven days after transmission.
Former New Zealand scrum-half Justin Marshall believes Wales will relish their underdog status at the World Cup.