Vicente del Bosque says Spain's second-half performance left "a bad taste in my mouth" even though his side hung on for a 1-0 win over Ukraine in Friday's European qualifier.
Alvaro Morata supplied Spain an early lead, but Ukraine found opportunities after the break with Iker Casillas stopping Ruslan Rotan on more than one occasion.
"We dropped off in the second half and that leaves a bad taste in my mouth," Del Bosque said.
"In the first half we played well and had chances. In the second we had chances for Juanfran and Cazorla but [our profligacy] cost us.
"[Morata] took his chance and scored. He met Koke's pass and gave a well-defined performance.
"Everyone did well. We got the three points, but our problems came in the second half."
Del Bosque said Spain will display a different look in their next match against Netherlands on Tuesday.
"There will be changes because there was no charge from the players," he said.
Even goal scorer Morata said he didn't think he played to the best of his ability.
Starting in place of the injured Diego Costa, Morata scored his first senior international goal in just his third appearance for Spain. But the Juventus striker played down his performance after the game.
"It was a very tough and physical match, but we managed to get the three points," Morata said. "It wasn't an easy game to play in. Our opponents were known for [this durability] and aggression and defended well.
"I hope to keep playing games [for Spain]. It wasn't my best game, but I'll keep working hard to get back here.
Morata wore the No. 7 shirt previously worn by Raul Gonzalez and David Villa, but the 22-year-old said he didn't feel the pressure to live up to his predecessors' standard.
"It is just another number to me. The most important thing is being here," Morata said.