It was not the introduction player or manager will have hoped for, particularly as Liverpool looked to be controlling the tie at 2-1 up against Bournemouth.
But a red card for Alexis Mac Allister forced Klopp to change his plans and send Endo on for his debut with 10 men and 30 minutes left to play.
The hope would have been to ease him in late on, but the Japan captain was instead required to shore things up as the Reds shifted to a 4-4-1 shape.
Endo kept things simple, completing 15 of his 17 attempted passes and winning his solitary tackle, as Diogo Jota‘s late goal ensured victory.
In Klopp’s eyes, though, it would be wrong to judge the £16 million signing’s output, having experienced a rollercoaster few days in leaving Stuttgart for Liverpool.
“Horrible,” was the manager’s verdict after the game.
“First game, coming in, I think we saw that Harvey [Elliott], who’s in a really good moment, came on in this game as well and was running left and right and the heart rate was 240.
“[It would have been] pretty much the same for Endo. The two days he had – no surprise!
“He did absolutely alright, but he came there and had absolutely no clue what to do.
“I spoke this morning with him about how, with 11 players, his position would look. A 4-4-1 was not involved in our conversation. That’s it now.
“I don’t think he slept a lot in the last two nights, so he came on and threw everything he had onto the pitch. That’s it.”