Richards was adjudged to have handled in the area for the spot-kick which led to Liverpool's first equaliser in a 2-2 draw at Anfield, a result which ensured the Merseysiders progressed 3-2 on aggregate.
He was penalised by referee Phil Dowd as the ball ricocheted off his leg and hit his arm as he attempted to block a Daniel Agger shot.
Richards, who had been playing in an unfamiliar role as one of three centre-backs, told City TV: "I think the referee made a poor decision for the penalty.
"It struck my foot and has gone on to my hand. I don't know what he expects me to do with that.
"But it was a decision. I won't say it has cost us but it has clearly played a massive part."
Richards remonstrated with Dowd at the time, as did a number of his team-mates, to no avail.
The 23-year-old said: "Everyone goes on about respecting referees but it is decisions like that that are sometimes costing us.
"I give my full support to the referees but with those sort of decisions, unless you are 100% sure, you shouldn't really be giving them."
City, the Barclays Premier League leaders, have been involved in a number of refereeing controversies in recent weeks.
Striker Mario Balotelli has now begun a retrospective four-match ban for violent conduct after referee Howard Webb told the Football Association he missed the striker committing a red-card offence against Tottenham on Sunday.
Captain Vincent Kompany has also sat out the last four matches after his contentious sending-off in the FA Cup third-round loss to Manchester United.
Gerrard's penalty, his second of the semi-final tie, restored Liverpool's aggregate lead after a stunning Nigel de Jong strike.
Edin Dzeko put City back ahead on the night but Craig Bellamy struck against his former club to send Liverpool through.