Louis van Gaal said he solved the problems posed by Preston North End in the FA Cup by changing the shape of his United side.
The Reds trailed to Scott Laird's early second half opener but hit back with three goals to earn a quarter-final spot against holders Arsenal at Old Trafford.
Ashley Young replaced the hard-working but ultimately ineffective Radamel Falcao on the hour mark with Marouane Fellaini switched to attack and the tactical move paid dividends with the Belgian scoring the second goal.
"I was reasonably satisfied with the first half" said the boss.
"We didn't give anything away, our ball possession was good and it's always difficult in such games especially with the long balls and the fights between the central defenders and Kevin Davies.
"It's always difficult but we only gave away once chance, I believe, in the first half.
"Then in the second half their goal was a little bit unlucky because it deflected but, after then, we showed an unbelievable team spirit.
"We changed the shape and that was the solution."
"I think we scored fantastic goals also but we were maybe a little bit lucky as well.
"It was good, I liked it, especially the second goal when Antonio Valencia crossed to the back post for Fellaini.
"I liked that and it was very pleasing for Valencia as we have trained on that a lot."
Fellaini remains an awkward and effective asset for defenders to deal with and the Belgian took his goal superbly after his initial header was saved by Thorsten Stuckmann.
"Yes, Fellaini has the power and body strength" added van Gaal.
"We can always overcome the pressure with him and he scores goals also.
"I played him for the first time in the position he wants to play but I have not always got a position for him and that's my problem."