Philippe Coutinho's stunning second-half strike extended Brendan Rodgers' side unbeaten run to 11 league games on Sunday, ensuring their charge for the top four continues to gather pace.
Victory moved Liverpool above Southampton into fifth, while dealing a huge blow to City's hopes of defending their title.
"That's a massive win for us," Lallana - who had two goals disallowed in the match - told Sky Sports. "We've got momentum, they're the champions and we deserved it.
"And the little magician [Coutinho] at the end, coming up with a wonderful goal. With players like that we'll go far.
"Our system has been causing problems for opponents, the manager said keep patient and find the space. We thought we were unlucky not to go in ahead at half-time.
"The lads showed character, many played extra-time in midweek [in the UEFA Europa League against Besiktas] and had a long flight but we showed we can keep fighting."
Henderson, who broke the deadlock with a superb effort of his own in the 11th minute, felt the win highlighted Liverpool's top-four credentials.
"We showed great character to stay in the game and Philippe won it for us with a fantastic goal," he added. "We have to keep going now, every game is big.
"Burnley [who Liverpool face on Wednesday] will be a tough game for us, but this result will give us great confidence."
