He confirmed at the pre-match press conference that Flekken, who joined Brentford in May from Bundesliga side Freiburg, would start the Group B game as the Dutch continue to struggle over a regular first choice for their team.
Flekken gets the nod ahead of Andries Noppert, who was a surprise choice for last year’s World Cup in Qatar where he went without having previously won a cap.
"Mark Flekken is going to keep goal because I think he deserves that, on the basis of being fit and in form for a long period. He's been in the squad for a while now," said Koeman.
It will be the fifth cap for Flekken but his first run out in the team since June last year.
"Andries had a great World Cup but was injured for a while. Then you have to make a choice," Koeman explained over dropping Noppert to the bench.
Since he returned as national team coach in January, taking over from Louis van Gaal, Koeman has picked Justin Bijlow and Jasper Cillessen for his four matches in charge.
Bijlow from Dutch champions Feyenoord is injured, however, while the experienced Cillessen has been dropped after a run of poor club form for NEC.
On whether Bijlow would return to the number one position once fit again, Koeman would not be drawn. "We'll have to see what situation will be then. He did keep goal in the Nations League finals (in June) but you can't say what will happen in a few months' time.
"I really want us to find a regular number one choice. All these changes of goalkeepers make things much more difficult," the Dutch coach added.
After the match against Greece on Thursday, the Dutch head to Dublin to meet Ireland on Sunday in a key match in the group, which also includes France.
