Fellaini was pushed forward from midfield to partner Wayne Rooney in attack as part of a switch that saw Radamel Falcao replaced by Ashley Young on the hour mark in Monday night’s FA Cup tie at Deepdale.
The Belgian scored United’s second goal – minutes after Ander Herrera had equalised – and van Gaal admitted his 6ft 4 man offers a crucial attacking alternative against stubborn opponents.
“I have used Fellaini a lot of times already – more than ever and he scores more than ever,” van Gaal said.
“He is a player that when we cannot beat the pressure with quality, we can always beat the pressure. That is a quality.
“Now I have played him for the first time in his (preferred) position but we have a lot of players who can play there.
“The whole team did it well because they believe also that we could do it with this shape.
“As trainer-coach or a staff, you can change the system but your players have to believe in it and perform it and that is not so easy and they do it and not for the first time. That is fantastic.”
Fellaini in turn praised his manager for introducing Young at the expense of £6m loan signing Falcao, who failed to record a shot at goal.
“We changed the game,” Fellaini said.
“We know the quality of Ashley Young; he can cross and it was an important substitution.”