MatchParis Saint-Germain v Liverpool: Champions League team news
Isak is part of the 21-man squad in the French capital for Wednesday night's encounter at Parc des Princes.
The Swedish striker returned to full training last week but was then left out of the Emirates FA Cup defeat at Manchester City.
An outing in Paris would be his first appearance since December 20, when he suffered an ankle injury that included a fibula fracture against Tottenham Hotspur.
Slot told reporters: "[Isak] finished close to a week of team training sessions now, so he can play a part. Otherwise, I wouldn't take him.
"I decided not to take him to the Etihad because we felt in talks with the player, medical, performance staff that he wasn't able to get a performance out of him yet.
"And since he's with us now, we do think that we can get a performance out of him now. But not to start – that [would] be clear."
With Alisson Becker still sidelined, goalkeeper Giorgi Mamardashvili is in line for his sixth Champions League outing.
Stefan Bajcetic, Conor Bradley, Wataru Endo and Giovanni Leoni are the Reds' other absentees.
A yellow card for Ryan Gravenberch, Curtis Jones or Virgil van Dijk would mean they are suspended for next week's return fixture.
PSG issued an update on their squad's fitness on Tuesday afternoon.
Bradley Barcola missed their two league matches either side of the March international break because of an ankle sprain.
The hosts' medical report said the forward is "back in full training and is working his way back to full fitness", though head coach Luis Enrique cast doubt on his involvement against the Reds.
"We want him to be in the best condition to play," said Enrique. "It is down to Bradley Barcola to say when he is ready."
Elsewhere, Fabian Ruiz has been sidelined since January due to a knee injury and is continuing with an individual programme, while Quentin Ndjantou remains unavailable.
Khvicha Kvaratskhelia and Nuno Mendes would be suspended for the second leg at Anfield if they are booked in this game.
On the tie, Enrique said at his pre-match press conference: "Who the favourites are is not important. Or who you say are the favourites.
"In this type of competition and this type of match at this type of level, it is impossible to have favourites. I don't believe we are the favourites.
"And if you remember last year, everybody said the favourites were Liverpool and the team that got through was Paris Saint-Germain. So, for me, there are no favourites.
"We will try to have more possession in the first leg and [in] Liverpool. We know beforehand how difficult it will be. They are a really good team with many quality players and a really good coach. It will certainly be tough."
Last match
PSG 3-1 Toulouse (April 3): Safonov, Hakimi, Zabarnyi, Pacho, Hernandez, Zaire-Emery, Beraldo, Kang-In, Doue, Kvaratskhelia, Dembele.
Manchester City 4-0 Liverpool (April 4): Mamardashvili, Gomez, Konate, Van Dijk, Kerkez, Gravenberch, Jones, Szoboszlai, Wirtz, Salah, Ekitike.