United returned to winning ways and ended a three-match losing run with victory at League One Northampton and set up a tie with local rivals City in the fourth round of the EFL Cup.
Manager Jose Mourinho could not afford any more slip-ups and it was eventually a smooth passage for the Reds, assisted by highly-rated Cobblers keeper Adam Smith who endured a nightmare evening and gifted the visitors two goals.
United started brightly and went close twice early on through captain Wayne Rooney and Ashley Young- on his first start of the season- but the Reds soon made the pressure tell.
Smith picked up a back pass in the 17th minute and, when Rooney's resultant free-kick was blocked, the recalled Michael Carrick fired in the loose ball with aplomb.
United were then denied a second goal on the half hour when Tim Fosu-Mensah burst into the box to head Morgan Schneiderlin's diagonal ball against the bar.
Rooney was on hand to nod home the rebound and seemingly double his side's lead only to be denied by the offside flag.
Alex Revell hauled the hosts level at the break after Daley Blind had fouled Sam Hoskins to leave the third tier side sensing an upset- but this was not a night for a shock.
Ashley Young's dangerous cross-shot almost found its way in and Schneiderlin went close from distance before Mourinho introduced Zlatan Ibrahimovic and Ashford ten minutes after the break in an attempt to go for the jugular.
Ibrahimovic nodded narrowly wide and Herrera hit the post with a 20 yarder but, just when it looked as though it would be another night of frustration, the Spanish playmaker restored the visitors lead.
Rashford collected the ball on the left and teed up Herrera who was not to be denied this time as he thumped the ball home past Smith with an excellent finish.
Rashford then turned from provider to scorer with 15 minutes to play as Smith completely mishit a clearance and allowed the teenager to roll the ball into the gaping net and put the result beyond doubt.
Alfie Potter had a late chance to set up a frantic finale late on but the Reds comfortably held out to return to winning ways ahead of the visit of champions Leicester on Saturday.
Overall team performance: 7/10
United Faithful Man of the Match: Ander Herrera