Frimpong took to Instagram Stories shortly after the squad announcement.
"Unfortunately it wasn't to be this time, but I'll be at the front cheering the boys on this summer," he wrote.
Koeman's reasoning was straightforward: a persistent injury record throughout the season made the 25-year-old an unreliable option for a tournament that demands physical continuity across six or more weeks.
"It is the pattern of this season that he has been injured regularly," the head coach told reporters.
The decision was a sharp one. Frimpong had been a consistent presence in the Dutch squad throughout the qualifying cycle and the Nations League, missing only when injured. He made 15 international appearances in total, scoring once, and was part of the squad for the European Championship in Germany two summers ago.
The season at Liverpool was a difficult one. Frimpong joined the club from Bayer Leverkusen and found himself caught between positions — not fully at home at right-back and not productive enough in a wider forward role to displace Mo Salah's successor. He made 35 appearances across all competitions, contributing two goals and two assists.
Koeman's squad includes Virgil van Dijk as captain, along with Liverpool teammates Ryan Gravenberch and Cody Gakpo. Denzel Dumfries and Arsenal's Jurrien Timber provide the right-sided options Frimpong would have occupied.
The Netherlands are in Group F alongside Sweden, Japan and Tunisia.