Roberto Martinez recorded a 2-0 loss in his first match as Belgium manager, and has called it a painful loss.
David Silva scored both goals for Spain, scoring on either side of halftime to hand Lopetegui a win.
Martinez believes Belgium has a lot of work to do, with their World Cup qualifying campaign starting against Cyprus.
“This game shows that we have a lot of work,” Martinez said. “We knew we would be tested.
“Sometimes you play friendly games in which you have little information, but it’s different now.
“The result is painful, but this is the reality. Against Cyprus becomes a different game, but every game is difficult.”
Belgium have often been described as having a ‘golden generation’ of players and Martinez conceded it is up to him to get more out of the “talent” available.
“As a team we need to find the right balance and we have to work hard,” the former Everton manager said. “With talent alone you become a winning team.
“We had the information we needed and we must now work hard. We played against a good opponent. That’s the level we want to achieve. We now know our weaknesses.
“Obviously it was not a fine spectacle for the fans. No one wants such a defeat, but it can also be a starting point.
“The talent is there, everybody knows that. We especially need to improve a lot mentally and work hard every day.”