The Rhinos are just a point clear of third-placed Hull FC following a 38-6 hammering by Wakefield Trinity and McDermott admits victory over St Helens is essential if they are to claim a home semi-final.
"I'm confident we'll get a reaction," McDermott said. "It was a poor loss last week. It was the first performance in such a long while where we lacked that bit of energy.
"Even in the Challenge Cup semi-final we had a crack but against Wakefield for the first time for a long time we lacked something.
"Nearly every time we've been beaten this year, we've won the week after and had a real good performance. We will get up for this game, for sure.
"It's a really important game. Top four is great - and we're not guaranteed that yet - but we want second spot.
"Had we beaten Wakefield last week, we would have been in a far better position to secure top four but it's all on now."
Friday's match will be the last in front of the famous South Stand before it is demolished as part of a major stadium redevelopment and the club plan an evening of celebration which McDermott believes could inspire his players.
"It's a big thing, it's like retiring a very old but very influential player," McDermott said. "It's been such a significant part of Headingley Stadium for ever.
"I know that in times when we've shot our bolt in games and haven't had much energy left, they have got the drum banging and start singing and it lifted us.
"As an opposition player, I never liked playing in front of it. By the time I became a professional player I had done a few things in my life and thought I was reasonably tough but it gets intimidating. When 5,000 people are chanting very clever chants at you, it does affect you."
Only two points separated the teams in each of their meetings in the regular season and St Helens coach Justin Holbrook, whose side slipped up at home to Hull last week, is anticipating another close-fought encounter.
"It will be a tough game for us with the event around the South Stand, but they are coming off a loss too," Holbrook said. "Their playing group will be hurting a fair deal."
McDermott has made just one change, axing full-back Ash Golding in favour of Jack Walker, who turned 18 last week and is battling with his rival for the number one shirt for 2018.
"If you don't reach a level of performance, you miss out and it's tough for Ash," McDermott said. "He's not happy but he understands.
"Jack's played a few games this year and been good. Who knows who will be the regular one for next season but I do know it will be based on form so we're onto a winner."
Elsewhere on Friday, Hull and Wigan Warriors will both be hoping to maintain their push for the top four while at the same time avoid injuries a week before they meet in the Ladbrokes Challenge Cup final.
Third-placed Hull will be close to full strength for the visit of Huddersfield Giants, whose coach Rick Stone is not expecting their opponents to be distracted by their trip to Wembley.
"Going into the big game like they are, I think they'll want to keep momentum," Stone said.
Salford Red Devils are aiming to repeat their notable victory at the DW Stadium in May - their first away win over Wigan for 20 years - and halt a six-match losing run which threatens to wreck their semi-final hopes.
The Warriors beat the Red Devils in the semi-finals of the Challenge Cup only three weeks ago and can move to within a point of their rivals with victory on Friday.
"We'll show them a lot of respect," said Wigan coach Shaun Wane. "They'll get a win again soon, we just need to make sure it's not against us."