Manchester United manager Jose Mourinho felt Cobblers midfielder Jak McCourt should have been sent off in the EFL Cup third round tie at Sixfields on Tuesday night.
McCourt was booked by referee Stuart Atwell for a challenge on Memphis Depay in the 47th minute of the match.
Manchester United went on to win the game, scoring twice in the second half, through Ander Herrera and Marcus Rashford to set up a fourth-round clash with Manchester City.
But Mourinho was far from happy with Attwell’s decision not to red card McCourt for the tackle on Depay.
“The game could and should have been easier because normally it is easier against 10 men and we should be playing against 10 men very early in the second half,” the United boss said.
“But they fought, they played with their strength and they tried to create us problems, which they did sometimes.
“It was a difficult match, but overall it was a positive performance.”
On McCourt’s challenge, he added: “It is really a dangerous situation.
“I’m not speaking about the penalty (United weren’t given) against Manchester City, I’m speaking about a situation that could hand an important injury.”
Cobblers had gone in at half-time level at 1-1 after Alex Revell’s penalty cancelled out Michael Carrick’s eye-catching opener.
But United brought Zlatan Ibrahimovic, Marouane Fellaini and Rashford off the bench as they finally got the job done.
And Mourinho said: “We deserved to win.
“It was difficult, as I was expecting, but we were totally in control for the first 35 minutes and then we were again in control for the last 45.
“The team were in control from the beginning and the substitutions were a contribution.”