World No.3 England are already on the brink of elimination after suffering a 30-12 loss to Tonga in Perth on Saturday night.
Under the confusing group stage system employed for the World Cup, only two teams from across pools B and C will progress to the semi-finals.
If Samoa beat Tonga and avoid an upset loss to Papua New Guinea, that would likely leave Tonga and England on two wins - with Tonga going through on the strength of winning their clash.
If Tonga do England a favour by beating Samoa, then Tonga are likely to go through unbeaten, leaving Samoa and England on two wins apiece, barring any massive upsets.
Under that scenario, it would come down to points difference, and Samoa's 78-4 trouncing of France has put them well and truly in the box seat.
McDermott, who was coaching England for the first time in Saturday night's four-tries-to-two loss to Tonga, says it would be a trap to assume his team can simply wallop France and PNG over the next fortnight.
"We're not going to trip ourselves up trying to come up with a plan about scoring 50 points next week," McDermott said.
"It would be tremendously disrespectful to either France or PNG to suggest that we're going to rack a load of points up."
Playing with freedom in a bid to score big is also out of the question.
"You can't play with freedom - not at international level," McDermott said.
"You've got to keep hold of the ball. Trying to get fatigue into the opposition is a hard thing if you're dropping the ball like we did.
"I saw enough against Tonga to tell me that we're heading in the right direction."
World No.5 Tonga take on world No.23 Lebanon and world No.4 Samoa in their last two games.
Even if they lose to Samoa, Tonga are still a strong chance to make the semi-finals.
"It's is a good position. We came here to win, and we knew the importance of the game," Tonga coach Kristian Woolf said.
"It's a big thing to get up and beat tier-one nations, and it's only happened a few times ... so really proud of the group. It's really special.
"But we've got to go and get a job done against Lebanon, and then we know we've got a big game the following week against Samoa."
Tonga's main concern is a pectoral injury to five-eighth Latu Fainu, who was injured just 16 minutes into Saturday's match.
