Van Dijk has starred for the Merseyside giants since the start of 2018, but the Dutchman conceded what could have proven to be a crucial spot kick in Sunday's Premier League fixture against Manchester City.
Riyad Mahrez missed the resulting penalty, but the incident has only led to Koeman suggesting that the 27-year-old is "too laid back".
Koeman is quoted by The Mirror as saying: "I know him so well – and that means I know where all his little faults are. Virgil has to improve. Not a lot, but sometimes he is a little too laid back. That needs to change.
"There are moments as a defender when you should be no more than three feet away from the striker you are marking. and particularly in Van Dijk's case, because of the strength and power he has he is too laid back.
"On Monday, when he arrived in our training camp in Zeist, he could see from the look on my face that I was thinking about the foul he made when Liverpool played Man City on Sunday. The tackle cost his team a penalty and Virgil realised that he had made the wrong decision."
Liverpool possess the joint-best defence in the Premier League this season with just three goals being conceded in eight matches.