Armstrong, on a season-long loan at Coventry City, met Benitez at the club's training ground soon after his appointment last month.
Benitez chose not to recall the 19-year-old from the Ricoh Arena despite a pressing need for more goals at the relegation-threatened club.
Armstrong has been prolific for Coventry – he has scored 19 League One goals – and he found the net this week for England's Under-19s, who qualified for the summer's European Championships with a win over Spain.
“I’ve spoken with him (Benitez),” said Armstrong.
"And he just said ‘carry on for the rest of the season and do what you’re doing – get as many goals as you can'.
“At Newcastle I was used to playing striker or No 10, but coming here I’ve played a few other positions which has really helped me I think.
"Going back to Newcastle, I think I can play anywhere across the front four now.”
Newcastle are 19th in the Premier League and odds-on to get relegated.
And United's plight is "hurting" boyhood fan Armstrong, though he believes Benitez's side will pull through and survive.
"Being a massive Newcastle fan, it’s hurting me where they are at the minute,” he said.
"But I believe they can pull through with the quality they’ve got and the staff they’ve brought in.
“Every player is working hard every day and you have to be positive. If you think you’re going to get relegated, what’s the point in being there?”