Lee and Paik, who have come through the Catalan club's famous La Masia academy and are regarded as two of the most promising talents in Spanish football, have impressed at the ongoing Under-20 World Cup in South Korea.
Both players scored in each of the victories over Guinea (3-0) and Argentina (2-1) to help the Koreans reach the last 16, where they will face Portugal on Tuesday.
But while the wing wizardry of Lee and Paik's range of defence-splitting passes have lit up the U-20 World Cup, Stielike said senior international football was an entirely different game.
"Those two players have done really well in the group stage," the German told reporters on Monday. "But before the U-20 World Cup started I watched them play (K League side) Jeonbuk Hyundai in a friendly match.
"And it was clear that there was a big gap between the Under 20 and senior level. It's too soon to bring U-20 players into the national side."
South Korea have appeared at every World Cup finals since Mexico 1986 but their hopes of grabbing one of the two automatic berths from Group A of Asian qualifying for next year's finals in Russia hang in the balance.
They trail group leaders Iran by four points and lead Uzbekistan by just one, and, after playing Qatar in Doha on June 13, must face both teams in their final two matches.
(Writing by Peter Rutherford; Editing by John O'Brien)
