Alex Neil’s side moved off the bottom of the Championship with a 4-0 win over their near neighbours as they ended a 10-game winless run.
Tom Barkhuizen and Callum Robinson had put PNE two goals to the good against Latics who saw Darron Gibson sent off.
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Nmecha was then felled in the box by Dan Burn late on but despite his request it was senior figure Gallagher who made no mistake from 12 yards.
Robinson then completed what had turned into a rout with the last kick of the game.
“I asked him to take it but he’s been taking penalties for as long as he’s been here,” said Nmecha.
“There’s not much you can do as a young loanee!
“I think he had it in the back of his mind to give it me but it was 2-0 and that was to make sure of the win really.
“I would have liked it but he took the responsibility and scored so I can’t complain.”
It was second time lucky for Nmecha who should have won PNE an earlier penalty when brought down by Cedric Kipre in the first half.
“He took my leg and then touched the ball afterwards,” said the Manchester City loanee.
“The referee’s made a mistake which is fair enough but it’s a clear penalty for me.”
All in all it was another impressive display from the teenager after catching the eye at Aston Villa last time out.
Now he just wants to add a first North End goal to his good work leading the line.
“I thought I did well myself but I can still do better and get the goals which I haven’t got yet,” said Nmecha.
“That’s my aim. I just worked as hard as I could. On the ball I still think I can keep it a bit better but overall it was quite good.
“I’m just happy for the lads that we’ve got the win, it was much-needed.
“We played well in other games but didn’t get the result.”