The Chilean sprung the diminutive striker from the bench and watched as he scored City's fourth goal in the 4-2 win over the Bluebirds to maintain the club's stunning home record.
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The 25-year-old was playing for only the second time since injuring his calf in the pre-Christmas clash with Arsenal but took little time to settle after scoring on his FA Cup return against Blackburn Rovers in midweek.
"I'm not surprised," said Pellegrini. "I see Kun [Aguero] working every day. He's that kind of player, when he comes back from injury, it's easy for him to retain his condition."
Manchester City scored its 100th goal of the season when Edin Dzeko opened the scoring and Pellegrini believes his side is improving every week.
"It's very important for the team," Pellegrini said. "It's noy easy to score 103 goals by a little bit more than the middle of the season. I think the first half was one of our best 45 minutes of the season.
"The players feel comfortable with what we are doing. Every match we have chances to score lots of goals. I hope we continue in the same way. We are playing better than at the beginning of the season."
Pellegrini admitted his disappointment at seeing his side switch off and concede twice but won't become too concerned so long as the goals come at the other end.
"It's always better to concentrate," he said. "It's better not to concede goals but that's not my biggest worry at the moment."