Two late goals from Jesse Lingard and Romelu Lukaku sent the Red Devils into the fourth round of the FA Cup.
United were frustrated for the majority of the match, and speaking after the game Mourinho admitted that during the match he feared that his side were heading for a replay, as quoted by BBC.
“I was feeling that we were close to scoring and we never stopped trying.
“But when you get past 80 minutes and the goal hasn’t come, you fear the second match. But the team kept trying so I knew the goal could arrive."
Lingard scored yet again for United, and Mourinho praised the Englishman for his attitude on and off the pitch.
“It was a great shot by Jesse Lingard in the top corner, he’s in a moment of confidence where things go well for him. He’s a good professional and a fantastic kid in the dressing room."
The visitors did defend well throughout the game, with a little help from the woodwork on the odd occasion, and Mourinho praised the Championship side and claimed that the standard in England's second division is close to that of the Premier League.
Thought derby gave a great account of themselves this evening . Class prevailed in the end and now we can go and concentrate on getting out of the league— Niall Horan (@NiallOfficial) January 5, 2018
“Derby defended very well. They changed players as the Championship is important to them. Scott Carson played well and the post played well for them. They were close to a replay – but they didn’t need it.
"The Championship is coming in a direction where they are Premier League level, they just play in another competition.”
(You may also be interested in Man Utd 2-0 Derby County: Two Late Goals Send Red Devils Through to FA Cup 4th Round)
Up next for United is Stoke at home, however with the game ten days away much could change in regards of personnel at Old Trafford with the January transfer window currently open.