Young replaced Radamel Falcao with United 1-0 down at Deepdale and the team immediately looked more dangerous as they looked to get back in the game.
Fellaini thinks he can do a job for Louis van Gaal in attack and that Young’s introduction changed the match.
Speaking about the tie, Fellaini said: ”We knew it would be very hard but we win today and that’s the most important.
“We changed the game, we know the quality of Ashley Young, he can cross and it was an important substitution.
“I like to play in that role [no.10], I can score goals but the manager is the boss.”
United started the game slowly and Preston deservedly took the lead but van Gaal will be pleased with how his side got back into the game.
The experienced Dutch manager has made some unusual decisions so far this season but Young’s substitution certainly made a positive impact on the game.
United will face Arsenal at Old Trafford in the sixth round of the competition.