The Scottish left-back admits he was tired after playing for the first time since January 18.
"I was just knackered!" Tierney said.
"Yeah, it was a tough game and I wasn't sure if I was going to get the full game, but I did. It's 90 minutes under my belt and I'm happy for that and happy that the manager gave me the chance again."
Tierney added: "That's what happened, really. Against a team like City you can't afford to make the start we made in the first five or ten minutes or they're going to punish you.
"It's hard because nobody was planning to not start well, so it's hard to say exactly what it was but it was just something that can happen against a top team and they punished us for it today
"You try to think positively because you know you're coming up against a team like City, so you know if you go one-nil down it's not the end of the world because they're playing brilliantly right now.
"We always believed that we could get back in it and we had chances and some parts of the game where we were doing really well and we were unlucky not to get a goal today."