Frimpong and Isak both picked up issues during the first half of the midweek Champions League win at Eintracht Frankfurt.
Gravenberch missed the trip to Germany as a result of a twisted ankle, while Alisson has been sidelined since sustaining an injury at Galatasaray last month.
Speaking at his pre-Brentford press conference on Friday morning, head coach Slot said: “Jeremie is not in a good place.
“What I mean with that is he is definitely not going to play tomorrow or next week. [A] hamstring injury so that’s going to take a while.
“Alex, not too bad. [A] question mark for the weekend, so let’s see where he is then.
“And that’s the same to be said about Ryan. He didn’t travel with us [to Frankfurt], so today we are in again. Yesterday we were in as well but we had a day to travel. Let’s see where he is today.
“Alisson is not in the squad tomorrow. That’s also going to take a little bit longer.”
The Reds travel to Gtech Community Stadium for an 8pm BST kick-off in the Premier League on Saturday.