The 26-year-old, who was bought last year to be the crowning glory for Slot's team, has been unable to get anywhere near matching the hype or his eye-watering £125million (A$238m) fee since his acrimonious move from Newcastle.
Isak has only managed 16 appearances for Liverpool because of injury. The latest, to his ankle, included a fibula fracture.
The Swede's absence has been keenly felt and his place in the 21-man squad for the European Champions League quarter-final first leg at PSG on Wednesday could be a major boost for Slot's stumbling side.
Pressure is mounting on the Dutchman and his team after poor recent performances were compounded by Saturday's humiliating 4-0 loss at Manchester City in the FA Cup quarter-finals.
However, Paris St Germain head coach Luis Enrique is not reading too much into Liverpool's FA Cup humiliation and expects a "difficult" Champions League clash.
Isak returned to team training last Thursday and worked with his teammates on Tuesday morning before they flew to France.
With Isak unlikely to start, a lot of attention will be on Hugo Ekitike, the former PSG striker who has led the Liverpool attack for much of the season.
He joined PSG four years ago as a promising 19-year-old but struggled to make an impact in a team containing Kylian Mbappe, Neymar and Lionel Messi. He scored just four goals in 33 games in two disappointing seasons.
However, a move to Eintracht Frankfurt changed his fortunes and earned a big transfer to Liverpool. He has done well with 17 goals and has also broken into the France team.
"Normally I don't talk about players who don't play for PSG," coach Enrique said. "But after leaving PSG he did well in Germany and England. He was very young when he was here and he has improved a lot."
The tie is a repeat of last year's clash in the last 16 when Harvey Elliott fired Liverpool to a superb 1-0 away win, due mostly to goalkeeper Alisson Becker's inspired performance.
But PSG reversed the scoreline at Anfield thanks to Ousmane Dembele's goal, before winning 4-1 on penalties.
"Last season, we put in one of our best performances, but we lost. Liverpool were favourites, and PSG qualified. There's no favourite at this stage of the competition," Enrique said.
"It's important to look to control what we can control. Liverpool are a very high-level team and to think you're the favourites is the best way to make a mess of things.
"It's not important to know who's the favourite in this competition, in this kind of game. It's impossible that there's a favourite. For me, there isn't one."
With agencies
