Second-half goals from Eden Hazard and Ramires cancelled out early efforts from United pair Wayne Rooney and Javier Hernandez before David de Gea made a vital late save when Juan Mata appeared to be set to score the winner.
Chelsea will now host United in the replay which is sure to clutter their schedule even further but Benitez was happy they get another chance after dominating for large portions of the contest.
"We analysed the game and we deserved to win. The first half we made two mistakes and conceded two goals but in the second half we were on top of them and had a lot of chances so it was a good game," he said.
When asked if United were tired from their Champions League clash with Real Madrid, Benitez was quick to point out his players had been involved in a Europa League match with Steaua Bucharest later in the week.
"I thought you were joking, they play on Tuesday, we play in Romania on Thursday," he responded.
"They had everything in their favour, they are playing at home, they played on Tuesday, we are coming from Romania and had to travel again so we did a great game and I'm really pleased with my players."