City lost 4-0, and finished the game with ten men
Manchester City's 4-0 defeat to Barcelona was one of the heaviest of Pep Guardiola's managerial career.
Numerous defensive mistakes made the scoreline look more one-sided than it actually was. Goalkeeper Claudio Bravo was sent off for a deliberate handball outside the box when the score was still 1-0.
Despite City's display, Republic of Ireland assistant manager Roy Keane felt the performance was "brave", yet "stupid".
Speaking to ITV Sport, Keane remarked; "I think City's approach to the game tonight was very brave, but very stupid as well".
"A few of the goals were from player who were guilty of overplaying in the wrong areas. That's the principle in football for a number of years. Everyone loves to see good football... Clearly City got punished tonight for the number of mistakes they made."
Barcelona look all but certain to top Group C, with three wins from three games. Manchester City are in second place on four points, with Borussia Monchengladbach a point further behind. Celtic prop up the table with a single point.
Guardiola will get his chance to gain revenge on his old side next month, in the second of the back-to-back games.