"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", Mourinho told the BBC.
The breakthrough would eventually come on 84 minutes, with Lingard unleashing a cracking effort from outside the box after a lay-off from sub Romelu Lukaku.
David De Gea, Phil Jones and Nemanja Matić were awarded much-deserved rests in five changes made by José Mourinho, as Sergio Romero made his sixth appearance of the season - every one of those in cup competition.
Receiving the ball on the edge of the box, United's outstanding talent of recent weeks curled a majestic effort around a helpless Scott Carson in the Derby goal. The top two teams in the Championship table at the end of the season are promoted to the Premier League, while the third-sixth place teams compete in a playoff for the final spot.
FA Cup match report Manchester United v Derby County 05 January 2018
This match could've been seen as a last chance for Henrikh Mkhitaryan to fight for his place, playing in his favoured central position against lower league opposition, but once again he didn't have the best of halves. But it was new boy Virgil van Dijk that got the final word, heading in the victor with few minutes left.
Manchester United had the first attempt on goal through captain Pogba firing a straight shot at Carson in the second minute.
Paul Pogba kicked off proceedings by picking out Lingard from the left, but his shot deflected over the bar.
Ander Herrera was the next Manchester United player to come close six minutes later but the Spaniard failed to hit target from a set piece from Rashford.
Jose Mourinho remarked that the post "played well" for Derby as Man Utd secured a 2-0 win over the Rams in the FA Cup third round.
Egypt's Mohammed Salah named African player of the year Egypt was named National Team of the Year , alongside its Argentinian manager, Hector Cuper , who was voted Coach of the Year . He netted the stoppage time penalty against Congo, which sent the Egyptian team to the world cup competition.
Cold weather to last through weekend
Temperatures could rise Saturday night , heading up to 30 on Sunday , with a 50-50 chance of snow, the Weather Service said. Highs will top out around the 29-degree mark on Thursday in the Topeka area, according to the National Weather Service .
Samsung Galaxy S9 and S9
Samsung is also set to release other major products for its smartphone collection such as the most-anticipated Samsung Galaxy S9 . Ahead of the launch, a lot has already been leaked about the smartphones, including a number of alleged renders of the devices.
No hope of peace without Jerusalem recognition
The United States is the largest donor to the agency, with a pledge of almost $370m as of 2016, according to UNRWA's website. Trump's top negotiators in the peace process are his son-in-law Jared Kushner and Jason Greenblatt.
United States calls for urgent UN meetings on Iran
South Carolina's senior senator also criticized the response of Iranian President Hassan Rouhani to the protests. We all know that's complete nonsense. "The demonstrations are completely spontaneous", Haley said.