The Reds were twice pegged back by Palace before losing the penalty shootout.
Afterwards, Slot reflected: "The main issue, I don’t think there was a main issue unless if the other team misses two penalties and you still lose within five penalties, that of course doesn’t help. But I think in general, it was a game where we twice came one goal up and, in my opinion, we controlled the game until the moment they scored the 2-2.
"Then, all of a sudden, although we still had one big chance for Mo Salah, we could have lost it in the end as well. But until that moment, (at) 2-1, there was every reason for me to expect that we would win this game. But after the 2-2 we were maybe lucky that we could go to penalties."
We're conceding
Asked about lessons from the setback, Slot continued: "Yeah, but that’s not something new. I think in the whole pre-season we saw that we are able to create more and we are more comfortable on the ball, we create more opportunities, chances, we dominate maybe even more.
"Last season we had a lot of ball possession but that didn’t always lead to promising situations. Now, we are better in creating and getting promising situations than we were, in my opinion, throughout the whole of last season.
"But the other side is also true, that we have conceded four against Milan, one against the Japanese team (Yokohama F. Marinos), two against (Athletic Club) Bilbao and again two today. What made us really strong last season was we only won mostly by a margin of one goal and that had mostly to do with us keeping a clean sheet or as a maximum conceding one goal. Today we scored twice against Palace, last season we were not able to do that once – we beat them 1-0 and we had a 1-1 draw at Anfield. That’s the positive.
"But conceding two; against a good team, by the way, that has made it really difficult for everyone last season and I’m expecting them (to) this season again. A lot of balls passed into our last line and a lot of crosses. For me, I felt that we didn’t give away that many chances until they scored the 2-2 and then they were threatening us more."