The defender has been linked to a loan move with Brighton and featured in their friendly win at Hertha Berlin.
Klopp remarked: "Last night I had another talk with him. When I came in, I saw him for 20 minutes playing in the U21 and then I saw the injury and I thought, 'Oh my God'. Everybody told me what a wonderful player he is, what a great talent and all that stuff.
"When he came back, he was injured, when he came back he was heavy - a lot of muscles where you don't need muscles if you don't want to go to the beach or whatever. We had to work on this and now he's back. He lost weight, he's really fit, he looks sharp, that's right. [There are] still things to improve - [his] heading was really good today but he can improve, he needs to be a little bit more aggressive.
"In the full-back position he is unbelievably important for us now because Clyne had no pre-season so far; Trent is a fantastic player but also just 18 and offensively he is probably ready but in defence he needs to improve a lot. We need these different options and it's good to have him back."