It should not have been such a slog considering how strong the team was, although key men like David De Gea and Paul Pogba were left at home. There was still plenty of experience on the pitch, notably in the shape of an unimpressive Wayne Rooney.
Mourinho's men began the game in the right way and they could have taken an early lead had Rooney been sharper. They eventually broke through following a slack mistake from Smith, who picked up a back pass from Matty Taylor, gifting a free kick 12 yards out.
The resulting free kick from Rooney was blocked by the wall but Michael Carrick, who had been the most impressive player on the night, was on hand to stroke the ball in. It had been comfortable for United up until that point, but the first half took a turn for the worse as the visiting team's intensity dropped.
United experienced their first scare in the 38th minute when Kenji Gorre, once a part of their youth system, caused problems for a disappointing Tim Fosu-Mensah by turning on the edge of the 18-yard box and sending a deflected strike against the bar.
United's defence soon alarmingly conceded a penalty. The blame could be spread around as Marcos Rojo failed to head clear, Fosu-Mensah sloppily hesitated and Daley Blind then felled Sam Hoskins, allowing Alex Revell to score from the spot. "You're getting sacked in the morning" was the chant directed at Mourinho.
Early in the second half, Northampton dished out some rough treatment, but United stepped up a gear as Herrera hit the far post with one shot and scored off another with a fine strike, showing his first-team credentials once again. Rashford was then gifted his third to seal the win, but it was far from impressive.