The Brazilian, who missed the first month of the season, played the first half of the third-round tie at the King Power Stadium before being substituted at the break.
The 25-year-old was an influential figure in the first 45 minutes as Liverpool dominated, but they failed to make the most of their opportunities and were later punished when Shinji Okazaki and Islam Slimani scored in the second half to put Leicester into the next round.
After the match, when asked why Coutinho was withdrawn, Klopp told reporters: "That was the plan before the game. He needs match time but it was not the plan to let him play again 60 or 70 or 80 minutes. Forty-five is the limit. We need to get him fit but we cannot push him through."
Liverpool will return to the King Power Stadium this weekend when they face Leicester in the Premier League.