While there were seven teams being tipped as potential winners, any of whom could beat New Zealand and South Africa on their day, there were only three who could win three games in the finals series, he told the South Africa's Business Day.
Matfield was part of South Africa's World Cup-winning effort in France in 2007 and played 127 Tests between 2000 and 2015.
"There's always something to fear. If you look at how the World Cup is set up, there's seven teams that could beat each other on any given day," he said.
Of those teams Matfield felt only Ireland was capable of going through the finals process, along with South Africa and New Zealand.
"We've seen what Ireland have done in the past two years. They are the No.1 team, even though they haven't shown the same form in the past year.
"You never know though, as they could peak at the right time. Their coach Joe Schmidt is a clever guy. He's got an experienced squad that can beat any team. They're going to be a dangerous side," he said.
Looking at the All Blacks, Matfield said lock Brodie Retallick's recovery from injury would be important.
The All Blacks had managed to cope without him, but he remained the country's best forward, who brought more to the side than lineout work.
"Scott Barrett is there and he's a good replacement. He's been fantastic for the Crusaders as a number four lock. He doesn't have the same skill set as Retallick who gets the ball in the midfield for the All Blacks. He gives them width on attack, whereas Barrett is a hard grafter and probably has a higher work rate than Retallick.
"It's the Brodie skill set that he doesn't have from a decision-making and playmaking perspective. That's where Retallick is important for them," he said.