The midfield maestro agreed with manager Mauricio Pochettino that our start was costly City hit us with a quick one-two to go 2-0 up inside 25 minutes.
Christian Eriksen reduced the deficit before half-time and we piled on the pressure without creating clear-cut chances. City's third ended the contest on 72 minutes.
The result ended a run of six-successive wins and 14 matches unbeaten in the Premier League and most agreed it wasn't our day.
Mousa, meanwhile, was looking ahead to our Premier League clash at Brighton on Tuesday (7.45pm) followed by the FA Cup semi-final against Manchester Unted on Saturday (5.