Sadio Mane fetched the only goal on 73 minutes to hand the home side all three points at Anfield.
The reversal is the Eagles second of the new season following a shock 3-0 loss to new boys Huddersfield Town in the opener.
“It’s very disappointing to lose the match after playing so well and carrying out the game plan; to lose in that way is very painful," Fosu-Mensah said, as reported by his club's official website.
“As a team, we did well, were very compact and broke their play up very well, so to come away with no points is frustrating.
“It is obviously hard but this is the Premier League so it is what you expect, we are away to Liverpool so you know it’s going to be a tough game.
"It was always going to be difficult but after this kind of performance, we know there is plenty to build on.
“You play football to win games, so you go out there looking for three points and we showed great spirit and we know we have quality in this team, so it shouldn’t be a major issue; we will definitely be getting the results very soon."
Despite the setback, Fosu-Mensah is delighted by the first team opportunity in Frank de Boer's set-up having played all two games so far following a loan move from Manchester United earlier this month.
“I am 19 years old and so to be playing at this level is good for my development but I am the first to admit I have a lot to learn and am willing to do that," the former Ajax trainee added.
"I listen to all the advice I get on a daily basis from the coaching staff and look to improve my game as time goes on.
“It was important to put in a good performance but last week is behind us and you have to take each game as it comes along.”
The versatile man, who can play as a centre-back, fullback or in midfield, has been contacted by the Ghana Football Association for a nationality switch as he currently features for the Netherlands at youth level.