Bayer tightens gip on second place
Son scored in the 62nd minute, when he let fly from outside the penalty area to send the ball inside the top right corner. Three `Gladbach defenders failed to close down the South Korean.
''There weren't many goal chances. The game was going to be decided on a mistake and unfortunately we made it,'' Moenchengladbach captain Martin Stranzl said.
Max Kruse was denied an equalizer by Bernd Leno in the 77th, three minutes before the Leverkusen `keeper kept out another attempt by Juan Arango.
Two Leverkusen substitutes, one in each half, were booked for slowing down play.
''There's still room for improvement,'' Leverkusen captain Simon Rolfes said. ''Other teams will get better as well.''
Leverkusen was 10 points behind league leader Bayern Munich before the rest of the weekend's games, while Moenchengladbach remained provisionally fifth after its third defeat in a row since the winter break.
''It's a pity. A scoreless draw would have been fair,'' said Moenchengladbach coach Lucien Favre, whose side hasn't won in five games. ''We have one or two things to correct. We'll see. I have to turn it around.''
Bayern can restore its 13-point advantage with a win at Nuremberg on Saturday.