Guardiola rested six of his regular starting XI players due to their four-goal advantage from the first leg fixture in Switzerland and was left unimpressed with the complacency shown by his side.
After taking an early lead through Brazil international forward Gabriel Jesus, the Citizens allowed the visitors back into the game with Mohamed Elyounoussi equalising mid-way through the first half and Michael Lang netting a 71st-minute winner.
"In the second half we forgot to attack, we forgot to play," he said after the game, as quoted by BBC Football. "Just to pass the ball without movement, to pass and pass for its own sake is nothing.
"Any team that makes six changes may have problems but in the second-half, we thought we had finished the game.
"It just didn't happen. We just passed for its own sake. That's not football."
Despite the defeat, the runaway Premier League leaders advanced to the quarter-final stage of the competition with a 5-2 aggregate score.