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 (5.15pm).
"We didn't start so well," said Mousa. "Towards the end of the first half we started to play well and we worked hard but at the moment we're all disappointed. We wanted to win.
"In the last 20 minutes of the first half we started showing ambition, but it was a bit too late. We had to start much sharper. We have to focus on the next games now, important games ahead this week. We're disappointed but we can't be disappointed for too long because we have to move on."
On his form...
Mousa: "I feel good, I feel fit, this is most important for me."
On Jan Vertonghen's form...
Mousa: "Everyone can recognise that all season, not only now, he’s been great for us. That’s how we know him. He’s an unbelievable player."