Arne Slot says Liverpool's first-half performance in Saturday's win over Southampton must serve as a "warning sign", suggesting they must improve massively to have any chance against Paris Saint-Germain on Tuesday.

Four days on from their smash-and-grab 1-0 Champions League first-leg win in Paris, Liverpool threatened to drop points in the Premier League title race against the bottom club.

Will Smallbone's goal in first-half stoppage time stunned Anfield, but a Darwin Nunez strike and two Mohamed Salah penalties ensured the Reds ultimately claimed the points.

Slot, who was watching on from the stands as he served the second game of his two-match ban, made a triple half-time substitution after seeing his team struggle early on.

Alexis Mac Allister, Kostas Tsimikas and Harvey Elliott entered the fray at the break, by which point Liverpool had managed two shots on target from 29 touches in the Saints area.

Slot told Sky Sports: "I made clear how difficult of a game this was and that we had to play at different intensity levels than we played against Paris Saint-Germain. 

"Maybe we also have to get used to that level of intensity that we're going to experience on Tuesday again. 

"But in the end, I had to make some hard decisions, maybe at half-time, to create a certain anger within the players.

"That triple substitution at the start of the second half seemed to reignite the team. The three that came in did really well. That's the first thing, but I could see from the other eight a different attitude as well."

Liverpool struggled with PSG's intensity in Wednesday's first leg, relying on Alisson to make nine saves – the most on record (since 2003-04) by a Reds goalkeeper in a Champions League game.

PSG had 27 shots to Liverpool's two during that game, and Slot has warned Saturday's performance was a long way away from the level they will need at Anfield on Tuesday.

"I'm fully aware of the fact that if you play a season, you cannot play 38 times at the highest intensity or the best possible football you can play," Slot said. "Sometimes you have to find another way of winning a game.

"But the first half today is hopefully a warning sign because like I said, we've experienced it already in Paris – the intensity, the way they play, was very high for us on that night. 

"If I compare it with the intensity we played today, it's not one, two, three, four, five steps down... it's probably seven or eight steps down from the intensity levels of Paris Saint-Germain."

The Opta supercomputer gives Liverpool an 84.6% chance of preserving their first-leg lead to eliminate PSG, while their 26% chance of lifting the Champions League trophy is the most of any club.