All four are making encouraging steps with their rehabilitation but still remain unavailable for selection.
Speaking at a press conference on Friday morning, the Reds boss said: "Closer, yes, of course closer.
"Luis takes longer but Diogo, Bobby and Virgil, a couple of weeks and [then they] can join training, and then we have to see how quick it goes from there."
Klopp was also asked about the status of Arthur Melo, who has been sidelined with a calf issue since October.
He said: "I would say pretty good, so he’s running a lot. Two or three weeks, I think, maybe until he can join team training.
"But it looks good for a while already. So it was now a long injury and he needs a proper build-up. He was not a part of team training or whatever.
"Fabio Carvalho is obviously out. He has a little thing. Did anybody tell me that he can train today? I don't think so, but maybe he can train today but he will not be 100 per cent ready for the weekend.
"Millie was not in training for one day but he's fine – there was no injury reason. Nat has a proper, proper cold."