The German will be without Mohamed Salah and Roberto Firmino for the clash at Anfield and is considering his forward options, and he said Brewster could make his senior debut for the Reds.
Per Goal, Klopp said:
"He's ready. There is a big chance. It's nearly sure. It's guaranteed.
"The situations are always like this. In an ideal world, in a season you always have much more players than you can involve in a squad.
"When you're on the bench, you are an option to play."
Brewster suffered a serious ankle injury in January 2018 that kept him out for over a year, but he returned to action for the youth team in April.
The 19-year-old was also part of the 23-man squad that travelled to Barcelona for the first leg, which Liverpool lost 3-0, but he did not make the substitutes' bench.
"Rhian has been in just outstanding shape for the last three or four weeks, but the other players were there as well.
"It's just good so now it's really nice in a difficult situation that we can give him that spot in the squad and have him there to bring on as a striker."
It seems the youngster will start on the bench on Tuesday. If so, Klopp will likely hand a starting berth to at least one of Divock Origi or Daniel Sturridge.
Football writer Joel Rabinowitz gave the Belgian some credit after he kept the Reds' Premier League title hopes alive on Saturday with the decisive late goal in their 3-2 win at Newcastle:
As for Sturridge, he has played 818 minutes this season and also scored four times, including two coming against former club Chelsea and another against Paris Saint-Germain.
The Reds will be up against it as they bid to progress on Tuesday, as they'll need at least five goals if Barcelona get on the scoresheet.
Brewster has nothing to fear, and getting on the pitch in a game of this size will be invaluable experience for him—the kind of experience he has missed out on during his time on the sidelines.