Following the Blues' win over Middlesbrough in the previous round Rafa Benitez had argued that criticism of his appointment as manager was harming the team, but the negative reaction among the away fans was once again audible after United went two goals ahead at Old Trafford.
"Sometimes it doesn't help when the fans are not happy. When you give the ball away there is pressure," the midfielder told The Evening Standard.
"This is our job. Fans come to watch the football and we have a responsibility to pay them back and make sure we play well. Every game we try to do that.
"Sometimes it works, sometimes it doesn't. This is when we need them the most. I have been here for seven years and Chelsea fans are fantastic and have always supported us until the end.
|4/1||Chelsea are 4/1 with BetVictor to win the FA Cup
"I know they will continue to be great with the players and let’s hope we can all go through the rest of the season together."
Mikel and Eden Hazard replaced Victor Moses and Frank Lampard after the interval, with Hazard scoring the first goal shortly after and Ramires benefiting from Mikel’s introduction in a holding role to break forward and net the equaliser.
When asked if Benitez's choice of substitutions led to the turnaround, he insisted credit should go to the whole team and not just the manager.
He added: "Yes, you could say that but the team who were there were doing well. It is never easy to play Manchester United at Old Trafford.
"Maybe we just needed a moment in the game that could change it for us, we came on and it just did. So credit to the team I think.
"It was just the manager who spoke [at half-time]. The manager said we just had to go out there and put things right.
"There was no anger, we just had a normal chat. The comeback was just determination, we found it from nowhere. It just worked."
The replay will take place at Stamford Bridge, while the Blues are still in the Europa League and fighting for a Champions League place.
Mikel continued: "I hope we can keep this character and momentum going but sometimes it's quite difficult when the season is going like this.
"This result has come at a crucial time where we just have to start putting on good performances but the tie is not yet done. You can never write off Manchester United at any time. At Stamford Bridge we have to make sure we put this tie away."