Louis van Gaal is to quit as Holland coach following the 2014 World Cup in Brazil, with Guus Hiddink in pole position to take over.
Koeman assisted Hiddink in 1997/98 - a period that included the World Cup in France - and at one point had been heavily backed to succeed Van Gaal.
However, Hiddink has emerged as clear favourite with Koeman set to stay with Eredivisie club Feyenoord.
His agent Guido Albers claims Koeman was offered the assistant role by the KNVB but in quotes reported in ad.nl, he said: "Assistant coach? Come on.
"It is also a little below his level."
He added: "I called them (the KNVB)... And two weeks ago I called even to ask whether Hiddink was the first candidate.
"It is all very unfortunate.
"I have been in contact with the KNVB for three years as Ronald so wanted that job. But it did not happen that way.
"That is where Ronald is not hurt, though. He accepts that there is another candidate. It is finished."