The Reds had initially fought back from two goals down in the first half to level matters in the 80th minute through Dominik Szoboszlai, however, Cherries winger Amine Adli snatched an injury time winner.
"Frustrating, of course," said Slot after the match.
"Because conceding a goal is always frustrating but especially if there's no time left to come back into the game.
"I think it's safe to say they could have scored the 3-2 also a little bit earlier and what I mean with that is I think after we scored the 2-2, we were still trying but I think it's safe to say a few players of ours ran out of energy."
The result marked Liverpool's fifth consecutive league match without a victory in between good results in the FA Cup against Barnsley and the UEFA Champions League 3-0 win against Marseille on Wednesday.
"I cannot even criticise them for that because two days ago we had to play an away game in Europe," added the Dutch coach.
"We are the only team that played [in the] Champions League that has two days between in this time. After an away game [then] another away game against one of the most intense teams in the league."
The Reds will turn their attention to their fixture against Azerbaijan giants Qarabag FK at Anfield on Wednesday before hosting Newcastle United in a league clash three days later.
Michael Carrick put a dent in Arsenal's Premier League title hopes with a dramatic 3–2 win, claiming maximum points from his first two games in charge of the club.
Chelsea moved up into the Champions League places and Aston Villa drew level on points with Manchester City as both clubs won important away games in the Premier League on Sunday.
Pep Guardiola aimed strong criticism at match officials after Manchester City's 2-0 win over Wolves, despite the result ending a four-game winless league run and cutting Arsenal's lead at the top to four points.
Liverpool's 13-match unbeaten run in all competitions came to a dramatic end on Saturday as they suffered a 3-2 defeat away to Bournemouth, sealed by a stoppage-time winner.
