Liverpool have suffered a form lapse over the past month as their new signings like Alexander Isak, Hugo Ekitike and Florian Wirtz have struggled to settle.
But Van Dijk told Reach Sport: "I think the club have done brilliantly in terms of the additions that we have made but they are not really additions because obviously we have lost so much quality that was so important over the last few years.
"Lucho (Luis Diaz), Darwin Nunez, Jarell Quansah, Harvey Elliott; that's obviously a lot of quality that we have sold.
"We had to replace those and replace them with quality and we know the market nowadays, it is just difficult but the club did brilliantly in getting these guys over the line. Now it is time to work and create a very, very good team on the pitch to compete in every competition that we are in. That is the target."
The Dutchman added: "To retain the title after winning it is such a difficult thing. We didn't manage to do it after winning it last time (in 2020), Manchester City managed to do it but it is such a difficult thing."