South Korea fell 3-2 to Poland in their friendly match at Silesian Stadium in Chorzow, Poland, on Tuesday. It was their second straight defeat following a 2-1 loss to Northern Ireland in Belfast last Saturday.
Against Poland, South Korea leveled the score from two goals down but surrendered the winner to Piotr Zielinski in second-half stoppage time.
Son, who has netted 18 goals this season for Tottenham Hotspur, said his side should not have conceded two goals from the start.
"As for Poland's first goal, they sent in a good cross for (Robert) Lewandowski, who is such a good player, but we should not have allowed the second goal," Son said after the match. "At the World Cup, all teams are stronger than we are, and if we concede easy goals, we'll face serious problems."
South Korea are in Group F at the 2018 FIFA World Cup, along with Germany, Sweden and Mexico. The match against Poland was to prepare for their clash against Germany at the World Cup.
"We have to consider that Germany are stronger than Poland," he said. "If we concede goals like we did in the first half, we'll really have some trouble."
Son lamented that the national team ended the two recent tuneup matches with losses.
"It's positive that our strong mentality helped us score two goals later, but as a player, it's a pity that we couldn't get a good result in the end," he said. "It makes us think about our shortcomings instead of our strengths."
Son, who has 20 goals in 63 international matches, said South Korea need to reduce mistakes at the World Cup.
"I think little things make differences in football," he said.
"We have to accept that we're not superior to others and need to concentrate even on small things."
Against Poland, Son started as a center forward, flanked by Kwon Chang-hoon and Lee Jae-sung, in a 3-4-3 formation. After Red Bull Salzburg forward Hwang Hee-chan came on to the pitch, South Korea switched to 4-4-2 to operate a two-striker system.
"When we're using three forwards, I was left alone most of the time," he said. "I had to wait for the ball and had to create space for teammates."
Son said he felt comfortable with Hwang playing up front. Hwang later scored South Korea's second goal to make the score 2-2.
"Hwang has good movements, and he penetrated Poland's backline to create space," Son said. "If we can use the two-forward system well, I think our attack can be upgraded."
With his second World Cup less than three months away, Son said he'll try to stay fit.
"Not only myself, but all the players should avoid injury," he said. "We're all tired physically and mentally right now, but I hope all of us can make better efforts to prepare for the World Cup."(Yonhap)