Liverpool had produced a disciplined display against Villarreal, frustrating a side that had won all six of their previous Europa League home games.
The Reds looked on course to secure a well-earned draw until their defence was caught out by a ball over the top and Denis Suarez raced away before squaring for Lopez to poke home and give Villarreal a slender advantage.
“It is not a moment for disappointment,” added Klopp. “If the players want to be angry for 10 minutes, then do it and move on.
“To be honest, it is the first leg. My first thought was ‘sorry, but it is not over, you have to come to Anfield, too, and we will be ready’.”
Liverpool staged a dramatic recovery against Borussia Dortmund in the previous round at Anfield.
The Reds came from 3-1 down to win 4-3 on the night and progress to the last four with a 5-4 aggregate victory.
Klopp cautioned against expecting a similar performance against Villarreal, but added: “Things are possible with 1-0, it is not the biggest result in the world.
“Villarreal know a lot more about us and they will respect us. Now we have to find solutions to a few things, but I saw we are in a good way.”