The Reds are 3-0 down from the first leg heading into the return at Anfield and will be without star forwards Roberto Firmino and Mohamed Salah due to injury.
The absence of Salah and Firmino, who have scored a combined 42 goals this season, means Brewster will be given a place in the squad and could make his first senior appearance for the Reds.
"He's ready. There is a big chance. It's nearly sure. It's guaranteed," Klopp told reporters on Monday.
"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."
The 19-year-old only recently recovered from an ankle injury he suffered in January 2018 but Klopp is happy with his fitness and confident he can call on him.
"Rhian has been in just outstanding shape for the last three or four weeks, but the other players were there as well," he added.
"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."
While Liverpool are dealing with injuries and an intense battle for the Premier League title that will go down to the final week, their midweek opponents were able to rest their entire starting XI from the first leg when they lost 2-0 to Celta, having already won the Spanish league title.
But Klopp expects his side to go all out to try to claw their way back into the tie and reach the final for the second time in a row.
"Tuesday night will be the third game in six days and Barcelona changed 11 positions on Saturday, so that is a massive difference as well, of course," Klopp said.
"Together with our supporters it was a long season and there is at least a little chance to make it even longer.
"If not, these are the last two home games of an incredible season.
"I think we should celebrate that with a good performance on the pitch and a very good performance in the stands. That would be really nice."