There are few players in the world in the kind of form that Son Heung-Min is in at the moment. The Tottenham Hotspur forward has been impressive all season but has stepped it up a level with seven goals in his past four games, shooting his way out of the shadows cast by the likes of Harry Kane, Dele Alli and Christian Eriksen.
In a turn almost as swift as the one that left Everton right-back Jonjoe Kenny feeling dizzy at Wembley in January, the South Korea star has gone from being the most underrated to one of the most feted players in the English Premier League. Viewers back home have rarely enjoyed watching any of their European exports as much as Son, but the sight of the rapier-like forward slashing through defences in England and Europe presents a double-edged sword for Shin Tae-Yong, head coach of the South Korea national team.
While it's great to see your best player tearing it up in the biggest stages of world club football and earning all the confidence that gives, Shin is concerned. He expressed his worries on Monday as to what kind of state Son will be in when he arrives in Russia this summer, where Korea will take on Germany, Sweden and Mexico in a tough group.
"I hope Son can perform in the World Cup like he currently does for his club," Shin said in Seoul as he named his 23-man squad for March friendlies against Northern Ireland and Poland. "But I'm concerned that Son's red-hot form could cool down around the World Cup period."
The problem is one with which many national team coaches are familiar.
"For most Europe-based players, they are used to getting in shape in August before the new season starts. I'm really worried that Son's form might go down from May, with his concentration waning and him becoming physically fatigued."
If the fear of the effects of a gruelling European season is familiar for many coaches, it is a novel one in Korea, where the opposite is usually the problem. Coaches feel the need to warn those not playing in Europe about what it will mean for their future selection prospects: a good example is Park Chu-Young's seven minutes of league football over three seasons with Arsenal ahead of the 2014 World Cup.
Never has there been such a vital member of the national team in constant European club action before a World Cup. At the 2006 and 2010 tournaments, the main man was Park Ji-Sung. While the Manchester United midfielder was an important player at Old Trafford, he never played every minute of every game yet logged enough minutes to stay sharp without being jaded (though there was a niggling injury in 2006).
Son is by some distance the star of the national team heading into Russia. Given the Champions League, FA Cup and Premier League -- not to mention international friendlies -- he is having a very busy final third of the season. Now that Kane is sidelined following an ankle injury picked up during Sunday's win at Bournemouth, the extent of which has yet to be revealed, Son is likely to go from playing most of the time remaining this season to pretty much all of the time.
A tired Son will make Shin's job harder, especially as he has yet to decide how to build around his biggest weapon. A succession of national team coaches have struggled to get the best out of the player or find the right system. Now it is Shin's turn. At least he should get plenty of chances to see Son being played in a central role for his club with Kane injured.
Shin has experimented with the Spurs star up front alone and out wide. In a November win over Colombia, Korea produced their best display of the year -- not much of a compliment to be honest -- with Son playing in attack in a 4-4-2. The warmup games against Northern Ireland and Poland should provide some answers.
The World Cup is not the only tournament that the forward may feature in this year. Son's winter has been busy and his spring looks like being full on as well.
If Korea get to the knockout stage, his summer will extend into July. At some point later that month, training for the 2018 Asian Games (an under-23 tournament that allows three overage players) will take place. With Korean men usually having to start their military service by the time they are 28, this Jakarta tournament, to be held in August, offers the last realistic opportunity for the player to win the gold medal that will him earn exemption from the 21-month tour of duty. Son's participation, hardly ideal for Spurs -- don't even mention the 2019 Asian Cup in January -- is starting to be discussed in Seoul.
Yet that isn't a concern for coach Shin. His job is the World Cup and the sight of Son shining at a high level is a welcome one. If Kane returns quickly, it may not only be fans in England breathing a sigh of relief.
Asian expert John Duerden is the author of Lions and Tigers: Story of Football in Singapore and Malaysia.Twitter: @JohnnyDuerden.