The Reds won 1-0 at Bramall Lane on Saturday thanks to Gini Wijnaldum's goal.
Klopp said, "I saw [the last] three games of theirs so that means I knew as much as I could. I am very impressed, to be honest. Very impressed. The way they play, the way they fight. I was a manager at Mainz and when any team came to Mainz, we thought whatever they did nobody could stop us enjoying the riot. We went 1-0 [today] and the stadium was there, that was really good. They have a proper basis to be a really uncomfortable opponent, especially here.
"They have really good set-pieces, that makes life really uncomfortable, and they are good around throw-ins, so many good little things that give them plenty of chances to win a football game.
"Obviously the relationship between manager and players is good. Chris [Wilder] is quite animated; it is my first game against him and I thought I am loud, but he is really loud! The boys listened and they go for it. That was impressive. Of course, it is easier to enjoy when you've won the game but anyway, I am sure we will come here again."