The Gladbach manager felt his team matched its Premier League opponent for long spells but admits his side lacked concentration to be able to see out the game.
Borussia Monchengladbach coach Andre Schubert says watching his team succumb 2-1 to Manchester City from Sergio Aguero's 90th-minute penalty was a bitter pill to swallow.
Gladbach took a deserved 54th minute lead through Lars Stindl in its first home match in Europe's top competition in 30 years before Andreas Christensen put Nicolas Otamendi's attempt past his own keeper.
The host could have been ahead had it not been for a string of fine saves from Joe Hart - including Raffael's first-half penalty - and manager Schubert was left to rue missed opportunities after Aguero netted from the spot in stoppage time.