The winger has missed Liverpool's last two matches following his father's kidnapping last weekend.
Both of the 26-year-old's parents were abducted by armed men on motorbikes in their native Colombia. While Diaz's mother has since been rescued, his father remains missing.
Luis Manuel Diaz's kidnapping has sparked a major police and military rescue effort, though reports on Thursday evening suggest there is hope that the Liverpool number seven's father will soon be released unharmed.
After Colombian rebel group ELN (the National Liberation Front) were identified as the perpetrators, the organisation sent a message claiming that Diaz's father had been taken without the knowledge of its leaders. Local reports say the police are now preparing a co-ordinated meeting with the ELN in order to facilitate a handover.
Amid the distressing events, Diaz has remained in Liverpool and will be available for matchday selection once again when mentally ready.
Speaking in his pre-match press conference ahead of Sunday's trip to Luton Town, Klopp said: “He was in training two days ago, he had yesterday a session and will be part of the team. We must wait – if he feels right he will be here and train with us.
“The session he had with us, because then we were not here, you could see that when he is with the boys it is fine, it’s OK. But you could see as well that he didn’t sleep a lot.
“We have just to see how he is and then we go from there. The news from Colombia, I didn’t get them personally, I always get them forwarded. Everything that gives us a little bit of hope is good. We are waiting actually for the really good news, but that’s it pretty much.
“I cannot really say what we will do because we just wait where we can pick up the boy from and go from there. But it’s all about him, that he makes himself available or not, and I will not force anything.”
Diogo Jota started on the Reds' left wing in Diaz's absence in the 3-0 win over Nottingham Forest last weekend. After scoring Liverpool's opener, the Portuguese forward ran to the side of the pitch to display a 'Diaz 7' shirt in tribute to his teammate.