The Reds defender says his strong tackle on the Anfield villain was for the fans.
Making his first Premier League start in 22 months, the 23-year-old sent the England man sprawling with a crunching challenge inside the first minute of Wednesday night's clash to a rousing reception from Reds fans.
Liverpool went on to win the match 3-0, with Sterling, who made an acrimonious exit from Merseyside to join City in the summer, being substituted at half-time after failing to have an impact.
Flanagan, who missed the entirety of last season with a knee injury, said after the match that he made the challenge as he knew it would raise the atmosphere from Liverpool supporters still sore at Sterling's departure.
He explained, as quoted by the Echo: "It was something I had to do to get the crowd up. The crowd have been fantastic, always right behind me even in the dark days.
“Me and Raheem were good mates at Liverpool but now he’s gone to City it is just business on the pitch."
Flanagan, who coincidentally made his debut for the club as an 18-year-old in another 3-0 win over City in 2011, is a fan favourite at Anfield after his impressive performances at right-back in the 2013-14 season.
Nicknamed 'the Scouse Cafu' after the legendary Brazilian full-back, had just made his senior debut for England before being struck down by injury in 2014.
Jurgen Klopp has reportedly been impressed with what he has seen from the defender since his return to fitness, and Flanagan is likely to be offered an extension to his current deal at the club, which runs out in the summer.