Klopp confirmed on Monday that Salah (concussion) and Firmino (muscular injury) would not be available for the second leg of the tie at Anfield as they look to overturn a 3-0 deficit.
Brewster, an England youth international, shot to fame when he won the Golden Boot at the Under-17 World Cup in 2017 where he scored eight goals.
Liverpool's Firmino out of Barca visit, Salah at risk
He signed his first professional contract last year.
"He's ready, there's a big chance," Klopp told reporters when asked if Brewster would be involved.
"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 had been sidelined for the majority of the season with an ankle injury and returned to action last month for the club's under-23 team, playing two matches where he grabbed a goal and an assist.
Barca's Dembele a doubt for Liverpool trip with hamstring problem
Klopp said the youngster has regained match fitness to be considered for his senior debut.
"Rhian has been in just outstanding shape for the last three or four weeks," Klopp 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."
COMMENTS
Comments are moderated and generally will be posted if they are on-topic and not abusive.
For more information, please see our Comments FAQ