Summerville has become one of the breakout stars of the early World Cup, with his standout performances for the Netherlands sending his transfer value soaring and prompting glowing praise from head coach Koeman.
Summerville started against Japan before coming off the bench in the Netherlands' 5-1 win over Sweden, contributing a goal and an assist after replacing a struggling Donyell Malen at half-time.
Koeman explained his decision to substitute Malen, who had started the tournament as the central striker before being moved to the right wing.
"That's difficult," Koeman told NOS after the match. "But it's top-level football, and performances are expected as a team. Of course I spoke with Donyell about why he was playing on the right side, but with a lot of freedom to come inside.
"It's not a real right winger in possession who holds the line, so he had freedom to make his own choices there."
Koeman confirmed that Malen's lack of goals also factored into the decision.
"Of course. That's also logical, everyone understands that. But he's not the one paying the price for it, because maybe he'll be back up front in the next match," Koeman said.
The Netherlands coach was effusive in his praise for Summerville's impact.
"I think Cry Summerville is brilliant in different ways of playing from the right side. As a substitute, but also as a starter," Koeman said. "He brings speed, of course, but he's also so smart between the lines and keeps possession. I think he releases the ball at the right moment for one of the last goals, and sometimes he holds onto it instead. I find it a pleasure to watch."
Summerville's performances have not gone unnoticed in the transfer market. The 24-year-old, a rare bright spot in a difficult West Ham season that yielded five goals and four assists in 31 appearances, scored a spectacular curling effort against Japan on his first competitive cap.
Paris Saint-Germain have been tracking the winger closely, with sporting adviser Luis Campos already holding preliminary talks with West Ham United over a possible double deal that would also include Portuguese midfielder Mateus Fernandes. PSG had hoped to take advantage of West Ham's relegation and financial difficulties to secure both players.
Summerville's World Cup form has since attracted a host of Premier League suitors, with Aston Villa, Tottenham and Arsenal all entering the race, while Manchester United have also expressed strong interest. West Ham United had already set a base asking price of €50m before the tournament began, a figure that has only risen following his goal against Japan.
The growing list of suitors has already cooled interest from Serie A, with AS Roma and AC Milan reportedly priced out of a deal that now appears to be developing into a battle between PSG and several of England's wealthiest clubs.