Before and after victory over Bournemouth on Friday night, Van Dijk has admitted he'd like to see a new centre-half added.
After the 4-2 win, Van Dijk said, "You can say you have to accelerate it (improvement) but that is down to the market.
"We have to focus on what's here and now. I think it (defensively) was much more improved than the last game but there is still work to do.
"The manager already said we are playing with a right-back (Jeremie Frimpong) who came from the Bundesliga, is 24 years old and is trying to get used to how we defend, we play with Milos (Kerkez) who is a very young full-back, very eager and full of energy, who is trying to get used to the way we play.
"It needs time, that's absolutely normal, but we also played without our six, Ryan Gravenberch, who missed out (with suspension). We have to keep working, that's the main thing, trying to keep clean sheets.
"We conceded two goals in a transition moment where we have to do better. That's what we have to work on."