The Reds won 4-1 thanks to goals from Dominik Szoboszlai, Jeremie Frimpong, Florian Wirtz and Hugo Ekitiké. However, Barnsley struck through ex-Liverpool youth teamer Adam Phillips, who took advantage of a fluffed backheel by Szoboszlai inside the Reds penalty area.
Afterwards, Slot said: "I was happy with the goals we scored - they were nice goals. But I think it was a game that was tight for far too long because at 2-0 and then giving a goal away like that made it difficult it until 5-10 minutes before the end. So yeah, good goals but not enough chances from all the ball possession we had."
On Szoboszlai's mistake, he continued: "I don't think you should do that in an FA Cup game or League Cup, a friendly game or even a training session. It's a weird choice and I also have my opinion about it but I prefer to keep that to myself and speak about that with Dom."
On Wirtz's performance from the bench, Slot was happier: "If you come in for half an hour and you have a goal and an assist and a few other good moments, you can definitely say he brought something creative to the team and that we what needed today."
And on Liverpool's line-up on the night, Slot insists he showed the Cup tie respect.
"There was also no other choice because I wasn't able to play 11 other players - because I don't have them. As you have seen today, you have to respect this competition and respect Barnsley - because even with large amounts of starters they made it really difficult for us."