Sylvain Distin and Antolin Alcaraz were both released by Everton over the summer, while John Stones is set to be the subject of an improved offer from Chelsea this week.
The Toffees have therefore been linked with Van Dijk and, although Martinez would not discuss the Dutchman directly, he did admit the need to strengthen at the back.
“That’s the nature of this period, you are going to be linked with players,” Martinez said when asked about Van Dijk. “I don’t enjoy talking about other players when they are not at our club and I will never be disrespectful in that manner.
“Our squad is not complete. We are still looking for two positions and we need to make sure they fit exactly in the group in the manner we want and expect as footballers and people.
“Central defence is one of those positions. Two experienced centre-halves left the squad, so we need to fill those gaps.
“I’m still assessing two of the young players we’ve got in the squad, but maybe we need another new face at centre-half.
“We will wait as long as we can to select the person we want.”