Lescott - now at West Bromwich Albion - had played in every round leading up to the final but found himself on the bench at Wembley as Pellegrini paired Vincent Kompany and Martin Demichelis together in defence.
The Citizens ended up beating Sunderland 3-1 on the day, and the 32-year-old admits that the omission still hurts him.
"I played every minute, captained the semi-final then not playing in the final – that was the hardest part," he told The Mirror. "I played as many minutes if not more than anyone in the League Cup. He just read out the team and my name wasn't in there. That was hard.
"I was like, 'If that's the reason for him, why isn't that a reason for me?' That was the final thing. Mentally that was the hardest of times, hearing that explanation and thinking logically, 'How can that be the reason?'"
Lescott left City on a Bosman in the summer and ultimately joined the Baggies.