Firmino was only fit enough for the bench, coming on for the closing stages, whilst he was altogether excluded from the squad for Liverpool's dramatic win over Newcastle on Saturday evening.
Salah suffered a worrying knock to his head after a collision with Magpies goalkeeper Martin Dubravka, with the Egyptian being stretchered off with suspected concussion.
The boss provided an injury update on the duo, confirming them both to be unavailable due to injury.
“Both are not available for tomorrow," Klopp told reporters on Monday.
“It's a concussion [for Salah], so that means he would not even be allowed to play. That's it, that's all.
“He feels OK, but it is not good enough from a medical point of view, that's all. He is desperate and everything, but we cannot do it. That's it.”
The duo's absence will be a huge blow to Liverpool's hopes of reversing the result of the first-leg.
Klopp acknowledged how hard it will be against Ernesto Valverde's side, but urged his side to give it their all, saying "if we can't do it, let's fail in the most beautiful way."
"There's hope and it's football," he said. "We are not in a situation where we say it will happen 100% but it's football.
"Two of the world's best strikers are not available and we have to score four goals to go through in 90 minutes. As long as we have 11 players on the pitch we will try for 90 minutes to celebrate the Champions League campaign to give it a proper finish.
"If you chase a result you lose a bit of focus on the other part of the team. I still like how we tried and defended it apart from one situation. That's the plan. If we can do it wonderful. If we can't do it, let's fail in the most beautiful way.
"The result at Barcelona didn't help a lot, honestly. We tried all the way through that to score. We have that result and we have to deal with it. Let's give it a try with all we have."