Scott Laird put Simon Grayson’s side in front with a deflected effort but goals from Herrera and Fellaini put United in the driving seat at Deepdale.
Wayne Rooney eventually sealed Man United’s progression into the quarter-finals from the penalty spot and van Gaal was very pleased about Herrera and Fellaini.
When asked if he was happy with the duo’s performances, van Gaal said: “Of course, they are important when they score goals and these were important goals. I was very happy for them because they are not always playing. Now they have showed a lot.”
Herrera’s showing in particular will delight United fans after the Spaniard was finally included in United’s starting line-up.
He looked assured in possession and took his goal well, although he did get a slice of luck as Rooney looked to be blocking the Preston keeper while standing in an offside position.
Fellaini also caused Preston plenty of problems with his physical presence and height.
Both Fellaini and Herrera will be hoping to cement regular spots in the team as United bid to qualify for the Champions League and win the FA Cup this season.