Champions Liverpool twice had the lead, but lost 3-2 at Falmer stadium after Jack Hinshelwood jumped from the bench to hit a late winner for the hosts.
Afterwards, Slot said: "Two very good teams wanting to play football, not time wasting, no diving. Momentum changed in the game and I waited for us to score the third, that would have killed the game but they made it 2-2. Great game of football but not the result we wanted.
"These moments at this level do make a difference. Just before that miss (Mohamed) Salah gave a similar chance to Federico (Chiesa) and it was very well blocked. We played eighth in the Premier League and they were able to bring in (Kaoru) Mitoma. That shows you about the strength of this league.
"If you don't score the third one then it's hard to win a game at this level, especially away from home. We cannot miss chances like we did if you want to win the game."
Fringe players' chance
On his selection, Slot explained: "Played their part in certain games and played their part on the training ground. Today and the Chelsea were moments for them. They deserve playing time.
"Many of them have to go to the national team so it's important for them to have their playing time and that's why we've rotated a bit. That's not why we've dropped points to Chelsea and Brighton because they are good team.
"It's going to be a completely different game than the one tonight. You saw Brighton want to press us high and bring the ball out from the back but if Palace play like their cup final then we'll have more possession than tonight but it's very difficult to play through them."