Demba Ba's spectacular 49th-minute volley and a stunning Petr Cech save earned Chelsea a 1-0 win at Stamford Bridge to set up a Wembley semi-final with Manchester City on April 14.
Chelsea's attentions now turn to Thursday's Europa League fixture with Rubin Kazan - their third of six games in 16 days - before Sunday's match with Sunderland in the Barclays Premier League.
The Jekyll and Hyde Blues are chasing a top-four finish and Champions League qualification next term but that particular challenge was derailed by Southampton on Saturday before their recovery against United.
Benitez said: "We had a turning point two weeks ago. One game (at Southampton), it seemed that everything changed.
"The race for the top four will be until the end of the season. We're in the top four with one game in hand so we're in a very good position.
"We're in the semi-finals of the FA Cup, quarter-finals of the Europa League. It's a great season, at the moment. It could be even better."
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