Lee Kang-in's issue is definitely stamina

When playing against weak teams, he can focus on attacking,
but when the opponent is high-level, he also has to defend, and you can see his stamina dropping during transitions.
After running around to press and defend, he makes many mistakes and slows down when it's time to attack.
It's a shame; it would have been great if he had stamina on the level of Park Ji-sung.