The Seagulls dropped a place to 11th in the Premier League with the defeat, while Liverpool moved into the top four. Liverpool's goals came from Emre Can, Roberto Firmino (two), Philippe Coutinho and a Lewis Dunk own goal, while Glenn Murray had replied to make it 3-1.
Stephens said: "It was tough, we had a plan we'd worked on during the week, working on 4-5-1 and keeping the spaces close and it probably worked for half-an-hour but then after the first goal it goes out the window a little bit.
"We're chasing the game and overall it's quite disappointing.
"We're quite good at it normally and deny our opposition a lot of space and the gaps quite close. It's worked for us in several games this year but we'll look back on it during the week and see what we could have done better.
"They had a lot of possession and we ended up probably dropping too deep in the first half but for half-an-hour it worked for us and as soon as the goal goes in, it's disappointing after that.
"There's no surprise they've come here and scored five goals because the attacking threat they've got is up there with the best I've played against. We don't really take many positives but we'll look back on the video and look at what we can do better for next week."
Albion had their opportunities during the game and Stephens said: "We got into some good areas and on another day probably pull ourselves back into the game at 2-2 or 3-2 and we've got a chance then. But if you don't score in the crucial moments, you're going to be under the cosh even more."