* Substitute causes coach selection concerns ahead of opener
* Korea coach not worried about loss (Adds quotes)
By Ken Ferris
MIAMI, June 9 (Reuters) - Ghana substitute Jordan Ayew grabbed a hat-trick to stake his claim for a starting place in a convincing 4-0 win over a sloppy South Korea side in their final World Cup warmup on Monday.
World Cup quarter-finalists in 2010, Ghana scored twice in the first half through Ayew and Asamoah Gyan before the former grabbed two more in the second half for the Black Stars.
Kwesi Appiah's Ghana are in one of the toughest sections in Brazil with a June 16 opener against the United States before they play European powerhouse Germany and Portugal in Group G.
"I don't look at the scoreline but how we played individually and as a team," said Ghana coach Appiah. "We took our chances. Korea attacked well but you have to be cautious at the back.
"If my team had lost by four goals it would maybe help me to look at the mistakes and correct them before the World Cup."
Ghana lost forward Majeed Waris with a thigh injury in the fourth minute after a fierce challenge by Ki Sung Yueng, who was booked, allowing the 22-year-old Ayew to be introduced from the bench.
The substitute did not take long to make an impact as he gave Ghana the lead in the 11th minute with a deflected right-foot shot that looped over goalkeeper Jung Sung-ryong off Ki.
Ayew's older brother Andre had set up the chance by feeding him the ball on the edge of the area at the Miami Dolphins' Sun Life Stadium which was filled with only a few thousand fans.
South Korea did come close to an equaliser when Son Heung-Min hit the post 10 minutes before halftime, a missed chance the Koreans would rue because Ghana soon got a second goal through the lively Asamoah Gyan who tucked the ball past Jung after robbing Kwak Tae-hwi in midfield.
Gyan ran at the Korea defence and although Kim Young-gwon tried to get across to cover he was too late to stop the striker firing the ball into the bottom corner of the net.
Jordan Ayew then grabbed his second of the night with a right-foot shot into the bottom left of the goal with the Korean defence asleep again raising concerns for coach Hong Myung-bo.
"It's very difficult to come back from two goals down," Hong said.
Ayew, who is the son of former African Footballer of the Year Abedi Pele, saved the best till last when he ghosted into the area to expertly turn the ball home from close range.
Ghana, who had lost their previous warmup match 1-0 to the Netherlands, were eliminated by Uruguay four years ago in a shootout after Luis Suarez infamously stopped Gyan's penalty in the last minute of extra time with his hand and was sent off.
It was another defeat for South Korea, who will be playing at their eighth consecutive World Cup, after they lost 1-0 at home to Tunisia before leaving for their Miami training camp.
The face Russia, Algeria and Belgium in Group H.
"It was individual mistakes rather than the way the team was organised that cost us," Hong said of the defeat.
"There is time to get better for the World Cup and I will do my best to improve the team." (Reporting by Ken Ferris; Editing by Greg Stutchbury)