Pressure had been building on the Netherlands before their impressive 3-0 win over Romania in the last 16 – but this first-half performance was back to their old ways.
Passive, predictable and vulnerable, they struggled to deal with Turkey’s threat on the counter-attack and did not create anywhere near enough at the other end.
An immediate change was needed with Turkey backed by vociferous support in Berlin, whistling every spell of Netherlands possession and outsinging the usually vibrant Oranje support.
Koeman did not hesitate and was rewarded for his ruthless decision-making, hooking Steven Bergwijn at the break and introducing Weghorst.
The striker had an immediate impact, causing problems with his height in the box, intelligent runs and work-rate in defence.
After taking the lead, the Netherlands had to battle through a rough period with Turkey intent on finding a response which never came.
Goalkeeper Bart Verbruggen denied Kerem Akturkoglu with an excellent save late on, as De Vrij made crucial blocks inside the area.
Unused substitute Bertug Yildirim was sent off for something he said to the officials, and moments later Netherlands players fell to their knees with relief at the full-time whistle, the tournament’s joint-top scorer Gakpo collapsing to the floor in celebration.
But it was Weghorst who took the bulk of admiration from supporters as his inspired second-half performance made all the difference.