Liverpool have only got half the job done in their Champions League semi-final against Villarreal and the Premier League side must be prepared for the worst against Unai Emery's team in the second leg, manager Juergen Klopp said on Monday.

Liverpool beat Villarreal 2-0 at Anfield and have won every away game in the competition this season, but Emery has an 84% success rate as a coach in European knockout ties since 2010.

"We didn't win the games by sitting deep and counter-attacking," Klopp told reporters ahead of Tuesday's second leg in Spain.

"We tried to play as ourselves and we have to do that again. We have to be ready to suffer and sit back in moments, but not as a general approach."

Liverpool have lost only once this year as they seek an unprecedented quadruple of trophies.

"We must've done something right in the last few months. The problem now is everything is perfect, then the next game we start on the wrong foot and all of a sudden the game is different," Klopp said.

"We know we're in a good moment. But... what do you do if you lose 2-0 away? Why should I think about the past?

"Let's say Villarreal have the first shot on target, the crowd goes up for each challenge... that's how it is."

Klopp added that "maturity and experience" will be important but not decisive factors.

"They will try to play much more football than we allowed them in the first game," he added.

"Unai will surely try to adapt a few things. It will be really interesting. Maturity is important but not the only thing."

Klopp said forward Roberto Firmino has started running again after a foot injury but was still experiencing pain and will not play in the match.