United won this fourth round tie 6-0, having hit five goals before halftime.
Mellon said, "You can see the quality they have. It was no surprise to us. Some of the goals they scored, we wouldn't face that kind of quality [in League One].
"We got everything we needed out of this cup run. We knew we were up against a fantastic team - the players gave everything. We can draw a line under it and get back to the bread and butter [of their relegation battle].
"We were undone by a centre-back shooting from 25 yards into the top corner, that's the quality we were up against. The quality was a level above what you're used to."