The 29-year-old missed the first half of the season after suffering setbacks to a thigh issue but is in contention to start the second leg of the Round of 16 Champions League tie at home to Porto as Liverpool look to protect a 5-0 lead.
"Adam, absolutely was an integral part of the team," Klopp told reporters. "With the competition we have, for Adam it is still usually not a problem for him, because he is important to us.
"Life is to learn from your mistakes and we all have to learn. Adam, myself and the medical department -- we made this mistake. He looked fit but obviously he was not.
"Maybe two little setbacks, not the biggest setbacks, but in and out, in and out, and that makes no sense. So now we have to build with him. He is coming close."
Lallana's willingness to lead the "counter-pressing" style favoured by the Liverpool midfield makes him a crucial component to Klopp's tactics.
The former Southampton skipper will also be keen to re-establish his position in the starting lineup and regain full fitness in time for England's World Cup campaign in June.
"He is coming back and training good and getting more and more stable fitness levels," Klopp added.
"To be honest, I would love to have six European and nine Premier League games -- that would mean there are a lot of games to come. We need Adam 100 percent and we will use him for sure."
Following the Porto tie, Liverpool visit bitter rivals Manchester United in the Premier League on Saturday as the two vie for second spot behind runaway leaders Manchester City.
(Reporting by Hardik Vyas in Bengaluru; Editing by John O'Brien)
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