Roberto Martinez conceded a 2-0 loss at home to Spain on Thursday in his first match as Belgium's head coach was a "painful" result.
David Silva struck either side of half-time to seal a comfortable victory for Julen Lopetegui's side at the King Baudouin Stadium in Brussels, and Martinez's men were booed off at the final whistle.
Martinez accepted there is "a lot of work" ahead for his team, who get their World Cup qualifying campaign underway in Cyprus on Tuesday, but stressed the quality of the opposition.
"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."
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