The Reds' pre-season preparations have been affected by numerous players taking part in the World Cup, with finalist Dejan Lovren, England midfielder Jordan Henderson and goalkeeper Simon Mignolet the last to return.
Speaking to the club's official website, Klopp said that there is "still a lot of work to do" before facing West Ham United at Anfield on Sunday.
"We need to get back the freshness, for one thing, and the other thing we still need to work on is endurance. The boys are not all on the same level, you can see that," the manager said.
"We need to play the first game somehow. Then we have another eight days because we play on Sunday and Monday. That is another pre-season week, I cannot change that. That's the challenge we have now.
"First of all, we have to make sure we have a good team for Tuesday and then for Sunday, then we have to prepare the next one. There's still a lot of work to do."
Liverpool play their final pre-season encounter on Tuesday evening against Torino.