Preston manager Simon Grayson masterminded a famous FA Cup victory over United during his tenure at Leeds United in 2010 and he must have been dreaming of another shock win when Scott Laird put the League One side in front just after half-time at Deepdale.
United avoided an upset, though, as Ander Herrera and Marouane Fellaini marked their recalls to the starting line-up with goals before Wayne Rooney struck from the spot after he had been upended by Preston goalkeeper Thorsten Stuckmann to seal a 3-1 win.
Van Gaal opted to replace the ineffective Radamel Falcao with Ashley Young and employ Fellaini as a striker alongside Rooney, who started in a more familiar role as a striker rather than in midfield.
Young laid on the first goal for Herrera and Fellaini put United in front, so Van Gaal was pleased to see his switch pay dividends as United set up a mouth-watering quarter-final at Arsenal next month.
The former Netherlands coach told BBC Sport: "I was satisfied [with the second half performance]. But also about the first half I was really satisfied.
"It is always difficult in such games, especially with long balls and fights between the centre-halves and [Preston striker Kevin] Davies.
"In the second half the goal was a little bit unlucky, it was deviated also I think. But after then we showed an unbelievable team spirit.
"Then we changed the shape and that was the solution I think. We also scored fantastic goals, a little bit lucky also I think. The second goal [Antonio] Valenica's cross to the second post for Fellaini I liked a lot. I was very happy for Valencia."
